docs screenshots

There are some screenshot-figures (png output) in the docs that need
minor updates:

- histogram: ylabel is cropped because the yticklabels are too wide
- table demo: the table is croped on the bottom of the image
- scatter demo: x and y labels are unreadable (the _i); ylabel is cropped
- financial charts: xlabels are cropped
- basemap demo: yticks on the right are cropped due to the colorbar;
colorbar ticks are cropped, too
- polar plots: title is cropped