Replace step with stairs and correct subplot_adjusts

[oscargus]

Closes #109 Closes #111

Hard to make the subplot_adjusts figure look really good, but here is the update one:

Stairs illustration:

(Both are screen shots of the PDF.)

Not obvious which arguments should be used. I went with those that are (a bit) "special" for stairs.

[timhoffm]

Style suggestions:

• in the subplot_pars, make the arrows dark grey like the figure width boundary lines. This should make the figure and the annotations more distinct. Right now we have a bit of abundance of hard to distinguish parallel lines.
• in the stairs, maybe use some a number of steps that does not march the grid so that not all steps align with the grid, c.f. the original step example.
• do not use vertical lines at the edge of the plot. They are mostly covered by the spine but a bit shines through. This does it look clean.

[oscargus]

I just recalled this...

• The lines were in fact dotted, but now they and the arrows are all gray. Not so obvious if the "external" arrows should be "on top" of the boxes, but I do not think the alternative looks better...
• All other example matches the grid only uses the basic arguments (not matching the grid will require using edges). While possible, I thought one of the ideas to use stairs rather than steps was to avoid hassling with this aspect... (For example, we can adjust the widths of the bars as well, but maybe not suitable for a cheat sheet example.)
• I modified the plot so that the first and last segment is at zero.

[oscargus]

[rougier]

Could you align y position on the grod for the stairs plot?