Allow to use a non fixed axes formatter

Hello,

I am a Python user that came from using plain Matplotlib (not an R user switching to Python). I noticed today that if I take e.g this plot:

And zoom in:

The x & y ticks are fixed! I tried to figure out from reading the code whether I can change this, and I encountered this in plotnine's code:

https://github.com/has2k1/plotnine/blob/06653090d05f6cf9ce6fc9f554794e0b106ed274/plotnine/_mpl/ticker.py#L6-L16

Which is used here:

https://github.com/has2k1/plotnine/blob/06653090d05f6cf9ce6fc9f554794e0b106ed274/plotnine/facets/facet.py#L351-L352

I am not sure whether the graphics grammar allows modifying this behavior, but I won't mind doing it via Matplotlib's native functions. However, that turned out to be hard and frustrating, as plot.draw() returns a figure, but I can't modify the axes of it. Here's something I tried:

from matplotlib.ticker import (
    EngFormatter,
    AutoLocator,
)
for ax in plot.draw(show=False).axes:
    ax.xaxis.set_major_locator(AutoLocator())
    ax.yaxis.set_major_locator(AutoLocator())
    ax.xaxis.set_major_formatter(EngFormatter())
    ax.yaxis.set_major_formatter(EngFormatter())
plot.show() # Also tried `matplotlib.pyplot.show()` here too.

And it doesn't show anything. As a side note, this made me wonder: What is the benefit in returning the figure if it cannot be manipulated afterwards with plain-old Matplotlib? This is related of course to:

• https://github.com/has2k1/plotnine/issues/982
• https://github.com/has2k1/plotnine/issues/1009

Anyway, I managed to workaround this with the following ugly monkey-patching of ggplot class:

from matplotlib.ticker import (
    EngFormatter,
    AutoLocator,
)
ggplot_original__draw_breaks_and_labels = ggplot._draw_breaks_and_labels
def ggplot__draw_breaks_and_labels_with_my_ax_manipulation(self):
    ggplot_original__draw_breaks_and_labels(self)
    for ax in self.axs:
        ax.xaxis.set_major_formatter(EngFormatter(unit="Hz"))
        ax.yaxis.set_major_formatter(EngFormatter(unit="Hz"))
        ax.xaxis.set_major_locator(AutoLocator())
        ax.yaxis.set_major_locator(AutoLocator())
ggplot._draw_breaks_and_labels = ggplot__draw_breaks_and_labels_with_my_ax_manipulation

Not sure why the ticks are located in fixed positions just due to the major_formatter set above, but for sure the AutoLocator is needed too.

Start by reading the fixed formatter in plotnine/_mpl/ticker.py and how it is applied from plotnine/facets/facet.py. Trace ggplot's draw flow and the interaction between plot.draw(), plot.show(), and the axes exposed by the figure. Done should allow users to apply Matplotlib locators and formatters such as AutoLocator and EngFormatter so ticks update appropriately when zooming, without monkey-patching ggplot.