luci-app-statistics: provide defaul minimal scale for some graphs

Avoid cryptic "200m" axis items for really small values by setting
a default minimal y-axis max value for some of the conntrack, cpu,
cpufreq, load and ping graphs (mainly percentages). The axis
automatically scales upward if necessary, so this just provides
a minimal "at least" scale for the y-axis.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>