I'm afraid that ShinobiCharts doesn't support this directly. We do support 'box zoom' which draws a box sized by the users pinch, but then zooms in on the selection (unfortunately there is no way to turn off the zoom). You can look at this behaviour by setting:
chart.gesturePanType = SChartGesturePanTypeBoxDraw;
Alteratively you might be able to implement this behaviour by adding your own pinch gesture recogniser to the chart. You would then have to add/size/style a UIView to the chart based the users gesture.
I hope the above info helps!