Change position of heart rate labels on y-axis

The lettering of the heart rate numbers on the lefthand exis "fall off"
the left hand of the profile panel when the panel is a bit narrow. This
minute change moves the letters a bit closer to the Y-axis so that their
left-hand extreme is more or less in line with those of the depth-labels
on the Y-axis. This is a partial response to Ticket # 484.

See #484

Signed-off-by: Willem Ferguson <willemferguson@zoology.up.ac.za>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>