Add range limitation feature to QValueAxis

I have some use cases where none of the displayed series could have any data in a specific range of a one of the horizontal/vertical axes.

For e.g. I only display positive data with the series.

When zooming and scrolling of the chart is enabled it would be useful if these operations could respect a limitation set on the axes itself.

I am thinking about adding the following API to the QValueAxis:

• void setLowerRangeLimit(bool enabled, qreal limitValue = 0.0)
• void setLowerRangeLimit(bool enabled, qreal limitValue = 0.0)
• bool lowerRangeLimitEnabled() const;
• bool upperRangeLimitEnabled() const;
• qreal lowerRangeLimit() const;
• qreal upperRangeLimit() const;

At the moment I workarounded this issue by deriving a class from QValueAxis and keeping a list from the limited axes in my zooming/panning logic, however it could be more elegant if it would be builtin to the QValueAxis.

 Please let me know what do you think about this change. If I got positive feedback I have the willingness of implementing it.