fix issue with datapoints containing null values

Sometimes, graphite will send null as the value of last item in the
datapoints. This results in the graph line going to 0 at the end.
Workaround the issue by repeating the next to last value.
fix period value on dashboard change

When changing between dashboards, the period will change to the default
period, until the next update.
Fix this by removing "period = default_period", since it has already
been set initially.
allow setting y-axis maximum of a graph

allow setting y-axis maximum of a graph - there is no default value.