pybootchartgui/draw.py: allow moving process chart up and down

pybootchartgui/draw.py: allow moving process chart up and down

Substracting curr_y when determining the hight of the process chart is
wrong because the height is independent of the position where the
chart is about to be drawn. It happens to work at the moment because
curr_y is always 10 when render_processes_chart() gets called. But it
leads to a negative height when other charts are drawn above it, and
then the grid gets drawn on top of those other charts.

Substracting some constant is relevant because otherwise the box is
slightly larger than the process bars. Not sure exactly where that
comes from (text height?); leg_s seems a suitable constant and happens
to be 10, so everything still gets rendered exactly as before.

(From OE-Core rev: b6bb690728c329ae448f89a1b68298c6dd8a573a)

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>