Review many generated charts at once with a contact sheet

When iterating on a batch of matplotlib figures, tile them 2x2 per image with imshow and look at three sheets instead of ten full-size images. Overflowing labels, crowded legends and missing axis titles still show at that size, and it costs far fewer image tokens. Two fixes that came up: adding y-axis titles to horizontal bar charts with long category labels needs a wider left margin (about 0.27 to 0.4 of the figure width with a monospace font), or the title collides with the tick labels. And when figures get taller, re-read the PNG sizes and update the width and height attributes on each <img> in the blog, so the layout does not shift.