add option "time serie" option to the XYPlot widget


the autoscale option will make the graph too small when x axis is a time value.

![2014-04-30-065711_803x434_scrot](https://cloud.githubusercontent.com/assets/1732355/2837942/698aaba6-d027-11e3-8787-50583efcffa2.png)

Adding an option for time serie axis make the graph conform to autoscale and to m_maxShowPoints :
![2014-04-30-065658_803x434_scrot](https://cloud.githubusercontent.com/assets/1732355/2837935/51d37c36-d027-11e3-90a0-a7e9cc850bd6.png)

![2014-04-30-070746_1608x841_scrot](https://cloud.githubusercontent.com/assets/1732355/2837962/be69a848-d027-11e3-8eb7-8de4aa0e19db.png)