Add this functionality out of the box to navigator. The functionality made by these buttons(+/-) are more gentle that the navigator.
RadHtmlChart already has handles in the navigator that allow the user to change the data scope. The designed UI is not going to be changed at this point. If you need to incorporate a slider, you can use the chart's client-side API to modify the range http://docs.telerik.com/kendo-ui/api/dataviz/stock-chart#configuration-categoryAxis-select-from .
I forgot to add screenshot: http://screencast.com/t/K1PYuh0FM