If you have a chart with a long range of X values, and want to easily focus on a particular range, you can use a Chart::WAxisSliderWidget. You can change the size of the focused region by dragging the blue handles, and change the position by dragging the selected area. When using touch, the size of this area can also be changed using a pinch movement.
If a series is assigned to a Chart::WAxisSliderWidget, but is hidden on the chart, then it will still be drawn on the WAxisSliderWidget. Combined with the zoomRangeChanged method of Chart::WAxis, this can be used to have a rough version of the data to present in the WAxisSliderWidget, and as the user zooms in, present a more detailed version of the data.
The example below shows a rough version of the data in the slider, and as you zoom in, a more fine version will be loaded on demand of the zoomed in area.