chart: Fix format string issues in draw_tick callers.

chart: Fix format string issues in draw_tick callers.

The 'label' argument to draw_tick is used as a printf-type format string,
so callers must pass "%s" to safely use an arbitrary string as a label.

Also, hist_draw_bar was doing its own snprintf, but this commit changes it
to take advantage of that provided by draw_tick.