Implement the rest of the heartbeat.

The only thing that was missing was the size of the vertical axis. I'm
setting a small line on the bottom of the temperature axis, since both
have blue color, this will not make people think one is the other.

TODO: change the color to red.

Signed-off-by: Tomaz Canabrava <tcanabrava@kde.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>