title: Plotting column values on condition of other columns of dataframe using matplotlib against same x-axis
body: <p>I have a dataframe with 600 rows in the shape of :</p>
  
  <pre><code>rpttime         metric          value
  25/4/2018 15:45 UTIL_CPU        1.5
  25/4/2018 15:44 IDLE            74
  25/4/2018 15:41 REC_BYTES_S     0
  25/4/2018 15:47 ENT_CPU         100
  25/4/2018 15:44 ENT_CORE        1
  25/4/2018 15:48 TRANS_BYTES     92
  25/4/2018 15:43 PINNED          5425
  25/4/2018 15:48 PAGING_PAG      0
  25/4/2018 15:48 IOPS_IN         NULL
  25/4/2018 15:47 TRANS_BYTES_S   23484
  25/4/2018 15:43 PAGE_OUT        0
  25/4/2018 15:42 IOPS_OUT        10
  </code></pre>
  
  <p>I want to plot line plots for the "value" column on y-axis against "rpttime" on x-axis as time-series for each individual items in the column "metric". There are around 20 individual items in the column "metric". There are some NULL values in the "value" column which are to be omitted which is the row is to omitted for each NULL value in the "value" column. There are atleast one NULL value for each individual items of the "metric" column. All line plots for those 20 individual items are to plotted in one graph. How to approach this ?</p>