Just a few preliminary requests:

• can you make the PR self contained so we don't need to look up support 'markevery' in prop_cycle #8576? And not refer to the issue number in the title?
• The documentation is quite self-referential. I don't know what markevery is, and I can't quickly grok it from anything in this PR. You do reference where I could figure it out, but a short description of what markevery does would help the documentation.
• does this need a test of actually using the property beyond the example?

Hi, thanks for the response.

I've removed the issue number from the title and made the PR self contained with enough information about markevery and the issue this PR resolves.

If I understood correctly, using the property beyond the example. The functionality for markevery is already supported within matplotlib. This PR just enables support for assigning a cycler object with markevery to axes.prop_cycle through the use of rcparams.

Let me know if anything else needs to be done.

@salindersidhu Thanks so much. Crystal clear now! I'll try an review when you are done. If you don't get feedback in a few days, feel free to ping...

Done, all checks have passed!

OK< sorry if I'm a lazy reviewer, but time is time. Is there no kwarg way to specify markevery? If not, that seems more important than an rcparam.

Don't think there's a kwarg way to specify markevery. Would probably need to open a new issue to request this as a feature correct?

I get confused about the linestyle cycles. Maybe they are only implimented via rcParams?

Sorry about the delay, after reading this link it turns out that we can assign markevery using set.prop_cycle in a kwarg way.

Axes.set_prop_cycle(*args, **kwargs)

I've demonstrated this using the code below.

from cycler import cycler
import numpy as np
import matplotlib as mpl
import matplotlib.pyplot as plt

t = np.arange(0.0, 1.0, 0.01)
ax = plt.subplot(111)
ax.set_prop_cycle(markevery=[5])
ax.plot(t, np.sin(2*np.pi*t), marker='o')
plt.show()

Upon testing this with the current stable version of matplotlib (2.2.0), I get the same ValueError as using markevery with rcparams. However, this code works using the fix from this PR.

Are you sure you checked all the files in? This PR doesn't touch any files to do with plotting or lines, so I don't see how it could have fixed anything... Sorry for the long process!

The answer is found in this file /master/lib/matplotlib/axes/_base.py

That file contains a class called _AxesBase which contains set_prop_cycle(self, *args, **kwarfs). This function calls a set_prop_cycle function from the _process_plot_var_args class through self._get_lines, which is an instance of _process_plot_var_args master/lib/matplotlib/axes/_base.py#L1204

The set_prop_cycle function from the _process_plot_var_args class assigns it's property cycler by calling the cycler() function here /master/lib/matplotlib/axes/_base.py#L169

We can see that this cycler function is imported from rcsetup.py here /master/lib/matplotlib/axes/_base.py#L38

The cycler function in rcsetup.py validates the keywords from a dictionary containing a keyword and it's validator. This PR added 'markevery': validate_markeverylist to _prop_validators which is used by cycler.

This is the reason why the fix in this PR works for set_prop_cycle because it uses a cycler object from rcsetup.py

Great detective work! Not at all obvious.

Awesome, any ETA on when this will be merged?

It needs two reviews

Thanks a lot @salindersidhu and thanks for patiently walking me through your changes!