In addition to using the 'mouse-box' to zoom, you can use the 'mouse-box' to select an area of the plot. Selection mode is enabled by holding down CTRL while drawing a box.
The resulting mtag message will include both width and height...give it a try! If you want range-selection to be the default, you can use the "rubberbox_action" setting.
plot.change_settings({rubberbox_action: "select")
SigPlot allows you to specify one or more X-ranges to highlight. You provide a starting index, ending index, and alternate color. Enable the selection highlight and select a few regions to see highlighting work.
plot.add_highlight(layer, {xstart: x1, xend: x2, color: "red"});
plot.clear_highlights(layer);
SigPlot supports the ability to constrain the selection box to be a horizontal or vertical selection. By default, "box" mode is used but while drawing a box you can press the 'b' key to alternate between the various modes.
plot.settings({rubberbox_mode: "horizontal")