Python - Plotting vertical line
Assuming you know where the curve begins, you can just use:
plt.plot((x1, x2), (y1, y2), 'r-') to draw the line from the point (x1, y1) to the point (x2, y2) 
Here in your case, x1 and x2 will be same, only y1 and y2 should change, as it is a straight vertical line that you want.
I have a curve of some data that I am plotting using matplotlib.  The small value x-range of the data consists entirely of NaN values, so that my curve starts abruptly at some value of x>>0 (which is not necessarily the same value for different data sets I have).  I would like to place a vertical dashed line where the curve begins, extending from the curve, to the x axis.  Can anyone advise how I could do this? Thanks