Debugging Matplotlib (Part 7)

I will pick up today by discussing why the x-axis labels are different for the lower subplots presented in Part 6.

To clarify some terminology, I have been saying “x-axis labels,” which I think is adequately descriptive and perhaps even correct. In different online forums, I have seen mentions of “tick labels” and “tick locations.” The 1st and 22nd of each month are tick locations on a date axis. The tick labels are what get printed at those locations. For dates on a date axis, tick locations and tick labels are identical.

The best answer I received to the original question says that matplotlib (MPL) is probably doing with dates what it does with numbers: calculating evenly-sized intervals to fit the plot (based on first and last values). He reports tick locations at the 1st and 15th of each month, though, which makes more sense as “evenly-sized.” The 21 days followed by 7-10 days I get at the 1st and 22nd of each month are lopsided. Although I still lack explanation for the latter, I did find this SO post showing the same thing (no explanation given there, either).

With regard to this line:

     > converted_Fri_2017 = [d.strftime(‘%Y-%m-%d’) for d in Fri_2017] #list comprehension

Values lose meaning when converted to strings. MPL spaces strings evenly without regard to any numeric or date value.

String conversion works in this instance because tick locations = tick labels, but other cases could present problems. One such case would be non-fixed-interval trade entry dates. Another example would be a longer time horizon where too many tick labels may render the x-axis illegible. If left as dates (or datetimes: both worked the same for me) then MPL could potentially scale accordingly (see first sentence of paragraph #3, above), but converting to strings robs MPL of this opportunity.

Much functionality remains with regard to ax.xaxis.set_ticks(), ax.set_xlim(), ax.set_xticks(), ax.set_xticklabels(), ax.tick_params(), plt.setp(), AutoDateLocator, ax.xaxis.set_major_locator(MultipleLocator()) from Part 3, etc. The list goes on, and solutions are varied based on version. That is to say they may have worked when posted, but if subsequent versions have been released (especially with previous functionality deprecated), those solutions may no longer be suitable.

I do not plan to write an encyclopedia of all the available functionality. I will resort to picking and choosing based on any particular needs I have at a given time.