title: quiver plot in Python using 2D arrays
body: <p>Hi I am trying to use the quiver plot to create a vector field plot.  Here is my logic and approach, I first create the x,y coordinates for position by np.arange and use a step size of 0.1.  Then I mesh the grid for x,y.  Then I import the x component of the function Fx, and the y component Fy into python as .dat files.  The .dat files are each 2D arrays (just a square matrix).  I then use the quiver command for the meshed x,y coordinates and the Fx,Fy 2d arrays.  However the quiver plot output does not make much sense at all in terms of what I was expecting.  </p>
  
  <p>Is there a problem with my code that I am overlooking?  Does np.arange work if the step size is not an integer amount? I printed out all the arrays to manually check the data and everything seems fine.  </p>
  
  <p>Could it be that my four 2D arrays do not all have the same shape?  The two .dat files I import are each 40x40 square matrices.   Not sure if this is matching up well with the grid I meshed.</p>
  
  <p>Other than that, I am unsure as to what the issue is.  Any help or suggestions would be greatly appreciated.  I can add the data in my .dat file if that will help.  Thanks! ( I have checked all other examples on stack overflow for this problem and it seems my code is logically correct so I am very stuck)</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  data = np.genfromtxt('file1.dat')
  data2 = np.genfromtxt('file2.dat')
  
  nx = 2
  ny = 2
  x=np.arange(-nx,nx,0.1)
  y=np.arange(-ny,ny,0.1)
  xi,yi=np.meshgrid(x,y)
  Fx = data[::5] #picks out every 5 rows in the matrix
  Fy = data2[::5] 
  #print(Fx)
  #print(Fy)
  #print(xi)
  #print(yi)
  
  plt.axes([0.065, 0.065, 0.9, 0.9])
  plt.quiver(xi,yi,Fx,Fy, alpha=.5)
  plt.quiver(xi,yi,Fx,Fy, edgecolor='k',facecolor='none', linewidth=.5)
  
  plt.show()
  </code></pre>
  
  <p>EDIT: .dat files below as asked.  If there is a way to import the .dat file let me know, I realize this is a lot of numbers and formatted horribly.  Fx is listed first, then the Fy array.  I am expecting a very nice quiver plot in which I have some kind of circular pattern/ circular flow. The arrows should all form a clockwise and or counter clockwise flow.  </p>
  
  <pre><code>  -30.9032192      0.512708426      0.511251688      0.508112907      0.503038108      0.495766401      0.486015081      0.473499298      0.457935333      0.439051390      0.416606665      0.390406251      0.360321403      0.326310992      0.288441181      0.246901810      0.202013552      0.154238343      0.104165822       5.24933599E-02   0.00000000      -5.24933599E-02 -0.104165822     -0.154238343     -0.202013552     -0.246901810     -0.288441181     -0.326310992     -0.360321403     -0.390406251     -0.416606665     -0.439051390     -0.457935333     -0.473499298     -0.486015081     -0.495766401     -0.503038108     -0.508112907     -0.511251688     -0.512708426       30.9032192    
  
    0.640149713      0.648661256      0.646115780      0.638335168      -13.4731970      -13.0613079      0.587181866      0.561966598      0.533295572      0.501472771      0.466741979      0.429292738      0.389282435      0.346857786      0.302170664      0.255400449      0.206771404      0.156560570      0.105099753       5.27719632E-02   2.10129052E-08  -5.27718328E-02 -0.105099864     -0.156560570     -0.206771582     -0.255400449     -0.302170008     -0.346857607     -0.389282405     -0.429292321     -0.466741502     -0.501472294     -0.533295095     -0.561966538     -0.587181747       13.0613060       13.4731960     -0.638335109     -0.646115661     -0.648661256     -0.640149713    
  
    0.799892545      0.824215114      0.801061392      0.776797950      0.753669202      0.730814993      0.707295001      0.682291210      0.655105412      -8.68122292      -8.12608242      0.554765701      0.513439834      0.467435867      0.416336209      0.359773695      0.297508597      0.229575798      0.156477526       7.93530941E-02   6.53175791E-10  -7.93530941E-02 -0.156477645     -0.229576021     -0.297508597     -0.359773695     -0.416336179     -0.467435598     -0.513440192     -0.554765582       8.12608242       8.68122387     -0.655105233     -0.682291508     -0.707294881     -0.730815291     -0.753669143     -0.776797950     -0.801061392     -0.824215114     -0.799892545    
  
    0.940612555      0.983826339      0.933131218      0.884394646      0.842061043      0.804476202      0.769944012      0.737089813      0.704840183      0.672395170      0.639202237      0.604933023      0.569452882      0.532750905      0.494812310      -2.68859553      -2.16188312      0.365726620      0.304749787      0.205249593       6.78142031E-09 -0.205249622     -0.304749817     -0.365726680       2.16188359       2.68859553     -0.494812399     -0.532750905     -0.569453001     -0.604932964     -0.639202118     -0.672395170     -0.704840362     -0.737089515     -0.769943893     -0.804476202     -0.842061162     -0.884394407     -0.933131695     -0.983826339     -0.940612555    
  
    0.999167860       1.05166125      0.986028075      0.923735499      0.870001256      0.822448075      0.778727889      0.736939847      0.695574820      0.653458953      0.609715879      0.563743949      0.515199065      0.463976830      0.410177410      0.354019582      0.295616359      0.234412342      0.167968050       9.07804966E-02  -8.54922577E-10  -9.07804891E-02 -0.167968005     -0.234412268     -0.295616418     -0.354019672     -0.410177410     -0.463976830     -0.515199006     -0.563743949     -0.609715819     -0.653458893     -0.695574880     -0.736939907     -0.778727889     -0.822448075     -0.870001316     -0.923735559     -0.986028075      -1.05166125     -0.999167860    
  
  
    0.940612555      0.983826339      0.932870448      0.884094179      0.841758013      0.804004610      0.768958390      0.735091329      0.701199591      0.666386902      0.630052805      0.591893077      0.551910400      0.510422051      0.468044579      0.425626040      0.384017974      0.343483299      0.302600116     -0.377980769       8.43500270E-10  0.377980769     -0.302600116     -0.343483359     -0.384017944     -0.425625950     -0.468044549     -0.510422230     -0.551910520     -0.591892898     -0.630052805     -0.666386902     -0.701199770     -0.735090971     -0.768958986     -0.804005086     -0.841758251     -0.884094059     -0.932870448     -0.983826339     -0.940612555    
  
  
   0.799892545      0.824215114      0.807587028      0.790868759      0.775763810      0.761242151      0.746228993      0.729784787      0.711097538      0.689466000      0.664264023      -6.33222771      -5.70436525      0.561126649      0.514991641      0.460934460      0.396892428      0.320130050      0.227872163      0.119494393      -1.02303694E-08 -0.119494416     -0.227872089     -0.320129842     -0.396892160     -0.460934043     -0.514991641     -0.561126769       5.70436525       6.33222771     -0.664264023     -0.689466000     -0.711097836     -0.729784369     -0.746228993     -0.761242330     -0.775764227     -0.790868759     -0.807587445     -0.824215114     -0.799892545 
  
    0.640149713      0.648661256      0.658376634      0.663496077      0.663335323      -12.7135134      -12.2490902      0.630356669      0.608760655      0.581994295      0.550120413      0.513214111      0.471384048      0.424800932      0.373717010      0.318486720      0.259573966      0.197552294      0.133099481       6.69753179E-02  -1.07370708E-08  -6.69753179E-02 -0.133099481     -0.197552368     -0.259573698     -0.318486512     -0.373717397     -0.424800485     -0.471384078     -0.513214111     -0.550120771     -0.581994355     -0.608760655     -0.630356669       12.2490902       12.7135134     -0.663335383     -0.663496077     -0.658376753     -0.648661256     -0.640149713    
  
   -30.9032192      0.512708426      0.511251688      0.508112907      0.503038108      0.495766401      0.486015081      0.473499298      0.457935333      0.439051390      0.416606665      0.390406251      0.360321403      0.326310992      0.288441181      0.246901810      0.202013552      0.154238343      0.104165822       5.24933599E-02   0.00000000      -5.24933599E-02 -0.104165822     -0.154238343     -0.202013552     -0.246901810     -0.288441181     -0.326310992     -0.360321403     -0.390406251     -0.416606665     -0.439051390     -0.457935333     -0.473499298     -0.486015081     -0.495766401     -0.503038108     -0.508112907     -0.511251688     -0.512708426       30.9032192   
  </code></pre>
  
  <p>Now Fy array:</p>
  
  <pre><code> -0.205083355     -0.525830388     -0.552687049     -0.580741763     -0.609929502     -0.640149713     -0.671258569     -0.703064799     -0.735320449     -0.767719150     -0.799892545     -0.831412077     -0.861791074     -0.890495777     -0.916961849     -0.940612555     -0.960886896     -0.977269113     -0.989315629     -0.996686459     -0.999167860     -0.996686459     -0.989315629     -0.977269113     -0.960886896     -0.940612555     -0.916961849     -0.890495777     -0.861791074     -0.831412077     -0.799892545     -0.767719150     -0.735320449     -0.703064799     -0.671258569     -0.640149713     -0.609929502     -0.580741763     -0.552687049     -0.525830388     -0.205083355    
   -0.495766401     -0.496165156     -0.509083092     -0.549605310       13.5129404       13.0519953     -0.646288395     -0.672055602     -0.695797563     -0.717920899     -0.738660455     -0.758110344     -0.776252687     -0.792979062     -0.808119476     -0.821464479     -0.832787395     -0.841867268     -0.848508835     -0.852558434     -0.853919387     -0.852558374     -0.848508716     -0.841867328     -0.832787514     -0.821464896     -0.808119833     -0.792978704     -0.776252151     -0.758110642     -0.738660395     -0.717920780     -0.695797503     -0.672055602     -0.646288335       13.0519953       13.5129395     -0.549605191     -0.509083092     -0.496165156     -0.495766401    
  
   -0.416606665     -0.387658477     -0.370003909     -0.412325561     -0.451486528     -0.484789789     -0.512974977     -0.536900580     -0.557342112       8.73137856       8.12754345     -0.604040861     -0.616312325     -0.627466083     -0.637651145     -0.646887839     -0.655064702     -0.661947429     -0.667217672     -0.670547307     -0.671688557     -0.670547426     -0.667217493     -0.661947429     -0.655064702     -0.646887779     -0.637651086     -0.627466381     -0.616312623     -0.604041040       8.12754345       8.73137951     -0.557341993     -0.536900103     -0.512975276     -0.484789670     -0.451485991     -0.412325561     -0.370003909     -0.387658477     -0.416606665   
  
   -0.246901810     -0.228335708     -0.217398927     -0.246074528     -0.271431714     -0.291785061     -0.307664692     -0.319617361     -0.328106791     -0.333535194     -0.336277753     -0.336733580     -0.335400879     -0.333002120     -0.330682963       2.81363893       2.24033999     -0.348281264     -0.372185618     -0.395866930     -0.403591305     -0.395866960     -0.372185677     -0.348281264       2.24033999       2.81363893     -0.330682874     -0.333002120     -0.335400909     -0.336733490     -0.336277664     -0.333535045     -0.328106642     -0.319617361     -0.307664692     -0.291785270     -0.271431714     -0.246074289     -0.217398927     -0.228335708     -0.246901810    
     0.00000000      -3.97699699E-02  -8.22334886E-02  -9.01840925E-02  -9.43243951E-02  -9.68469381E-02  -9.79287177E-02  -9.75681171E-02  -9.57226083E-02  -9.23085213E-02  -8.71856511E-02  -8.01347122E-02  -7.08276853E-02  -5.87978214E-02  -4.34263758E-02  -2.40071025E-02  -4.12676527E-05   2.79203784E-02   5.66387177E-02   7.90976062E-02   8.76100808E-02   7.90975988E-02   5.66387326E-02   2.79204026E-02  -4.12871887E-05  -2.40071043E-02  -4.34263758E-02  -5.87978400E-02  -7.08276406E-02  -8.01346377E-02  -8.71856511E-02  -9.23085883E-02  -9.57226381E-02  -9.75680798E-02  -9.79286432E-02  -9.68469679E-02  -9.43244398E-02  -9.01841149E-02  -8.22335258E-02  -3.97699960E-02   0.00000000    
  
   0.246901810      0.149554759       5.41899577E-02   6.69130459E-02   8.30149651E-02   9.62892994E-02  0.106718197      0.114569001      0.119987577      0.122970015      0.123354375      0.120809816      0.114815064      0.104622498       8.91864598E-02   6.69886991E-02   3.55363674E-02  -1.02187870E-02  -8.21609423E-02 -0.177876130     -0.191068053     -0.177876085      -8.21608678E-02  -1.02187609E-02   3.55363339E-02   6.69886544E-02   8.91865119E-02  0.104622573      0.114814982      0.120810024      0.123354279      0.122969493      0.119987287      0.114568666      0.106718197       9.62890834E-02   8.30147490E-02   6.69130459E-02   5.41902333E-02  0.149555355      0.246901810    
  
   0.416606665      0.324635506      0.239433557      0.271107137      0.304715306      0.333829224      0.358776420      0.380251735      0.398895025      0.415270001      0.429880798      -6.52393579      -5.84947205      0.467720896      0.479777455      0.492111117      0.504699171      0.516976655      0.527697802      0.535157621      0.537844956      0.535157681      0.527697802      0.516976714      0.504699290      0.492111027      0.479777277      0.467720628      -5.84947205      -6.52393579      0.429880500      0.415270001      0.398895413      0.380252063      0.358776003      0.333829224      0.304715246      0.271106362      0.239433587      0.324635804      0.416606665    
  
   0.495766401      0.468931794      0.452914894      0.491556555      0.528390408      -12.8101072      -12.3052654      0.617275119      0.641844690      0.664552093      0.685565233      0.704941750      0.722658634      0.738638997      0.752775729      0.764953554      0.775063336      0.783014059      0.788738489      0.792190075      0.793342948      0.792190075      0.788738668      0.783013999      0.775063157      0.764953852      0.752775729      0.738638759      0.722658694      0.704941571      0.685565174      0.664552152      0.641844690      0.617275119      -12.3052645      -12.8101072      0.528390408      0.491556555      0.452914953      0.468931794      0.495766401  
  
  0.512708426      0.525830388      0.552687049      0.580741763      0.609929502      0.640149713      0.671258569      0.703064799      0.735320449      0.767719150      0.799892545      0.831412077      0.861791074      0.890495777      0.916961849      0.940612555      0.960886896      0.977269113      0.989315629      0.996686459      0.999167860      0.996686459      0.989315629      0.977269113      0.960886896      0.940612555      0.916961849      0.890495777      0.861791074      0.831412077      0.799892545      0.767719150      0.735320449      0.703064799      0.671258569      0.640149713      0.609929502      0.580741763      0.552687049      0.525830388      0.512708426   
  </code></pre>