title: Make reverse diagonals white in heatmap
body: <p>I'm trying to do something as seen on the image is given below,
  <img src="https://i.stack.imgur.com/fmMC6.png" alt="enter image description here"></p>
  
  <p>Just setting reverse diagonals white color is left. I couldn't set them as white. The chart takes integer values and I don't know what integer value is corresponding of white color.</p>
  
  <p>Thank!</p>
  
  <p><strong>Edited:</strong></p>
  
  <p>Here is the code;</p>
  
  <pre><code>import math
  from matplotlib import pyplot as plt
  from matplotlib import cm as cm
  import pylab
  import numpy as np
  from matplotlib.collections import LineCollection
  
  class HeatMap:
      def __init__(self, selectedLines):
          self.selectedLines = selectedLines
  
  
      def getHeapMap(self):
          figure = plt.figure()
  
          if len(self.selectedLines) != 0:
  
              self.map = self.createTestMapData(len(self.selectedLines),                len(self.selectedLines))
  
  
              maxValueInMap = self.findMaxValueInMap(self.map)
  
              x = np.arange(maxValueInMap + 1)
              ys = [x + i for i in x]
              ax = figure.add_subplot(111)
              ax.imshow(self.map, cmap=cm.jet, interpolation='nearest')
  
              '''
              Left side label of the chart is created according to selected values
              from a checkbox group.
              '''
              leftSideLabelSize = len(self.selectedLines)
              sideLabels = []
              for line in self.selectedLines:
                  sideLabels.append(line.text())
              pos = np.arange(leftSideLabelSize)
              '''
              Left side labels are set with the code below.
              '''
              pylab.yticks(pos, sideLabels)
              plt.xticks(pos, sideLabels)
              self.numrows, self.numcols = self.map.shape
              ax.format_coord = self.format_coord
  
              line_segments = LineCollection([zip(x, y) for y in ys],
                  linewidths=(0.5, 3, 1.5, 2),
                  linestyles='solid')
              line_segments.set_array(x)
              axcb = figure.colorbar(line_segments)
  
          return figure
  
      def format_coord(self, x, y):
          col = int(x + 0.5)
          row = int(y + 0.5)
          if col &gt;= 0 and col &lt; self.numcols and row &gt;= 0 and row &lt; self.numrows:
              z = self.map[row, col]
              return 'x=%1.4f, y=%1.4f, z=%1.4f' % (x, y, z)
          else:
              return 'x=%1.4f, y=%1.4f' % (x, y)
  
      def createTestMapData(self, xSize, ySize):
          resultMap = 10 * np.random.rand(xSize, ySize)
          #Setting reverse diagonal is here. Now it is set with zero but it gives blue.
          # I want it to be set as white
          for index in range(0, int(math.sqrt(resultMap.size))):
              resultMap[index][((math.sqrt(resultMap.size) - 1) - index )] = 0 
          return  resultMap
  
      def findMaxValueInMap(self, map):
          return np.amax(map)
  </code></pre>
  
  <p>The values are generated randomly at the moment. The code is above gives a gui like;</p>
  
  <p><img src="https://i.stack.imgur.com/K5nOf.png" alt="enter image description here"></p>