How to plot and work with NaN values in matplotlib

Question

I have hourly data consisting of a number of columns. First column is a date (date_log), and the rest of columns contain different sample points. The trouble is

Answer 1

I simply use this function:

import math
for i in range(1,len(data)):
  if math.isnan(data[i]):
    data[i] = data[i-1]