Plot of mean DLP over time y-axis auto ranges to negative dose values

[David Platten]

The auto ranging of the y axis goes negative (despite there being no negative data). Need to set a minimum allowed value of zero.

[David Platten]

Added floor to y-axis to prevent auto ranging from choosing to use a negative value as the minimim. Fixes issue #173.

→ <<cset 6b1d818e366a>>