Support for a continuous axis

[gardejo]

Summary

A horizontal (X) axis of materials chart (Line and Candlestick) is only discrete now. It should be also support a continuous axis.

Technical Notes

A continuous axis requires us to set the type of the first column is datetime.

See also

• API document.
• The 451st response of the 7th KCRDB thread

[gardejo]

This issue was completely resolved by the commit 4a7a63753a5ac62e1456534146a3605787bc7467.