Incorrect interpretation of maxValue / minValue when values are negatives

[glebiller]

It seems to have an issue when values are interpreted as Strings and values are negative. The comparison does not work the way it should, and the maxValue / minValue calculation is wrong.

To fix the issue, I have added a "parseFloat(value)" at first line of the method "TimeSeries.append".

I think it should be safer to include it in the code instead of making sure people are sending numeric values. When working with JSON this is not always true :)

[drewnoakes]

Issue #45 duplicated this issue, and provided a patch that has now been merged. I'll close this too. Thanks.