Changed derivative graph to show value per second

The derivative graph used to show value per ms,
I have changed it to show the value per second.
Added timestamp check for #safePlot

the safe plot method now checks if the timestamp is in ms or seconds.
If it is in ms it multiplies by 1000.
cassandra derivative graphs now use DerivativePlot class
added safeplot for latency graph
Merge pull request #114 from dzautner/fix-cassandra-tab

Fix cassandra tab