it seems you have resolved the issue yourself in the meantime - the diagram shows ft/mi when i look at it. In case you still see metrical units, it may be your browser cache.
I have set the elevation diagram units to ft and x axis grid units to mi but it still displays km. I also tried setting in the plugin shortcode with {gpxtrackmap}timp-half.gpx,edunits=ft,edxgridunits=mi{/gpxtrackmap} but its not working