I believe that the Chart's Slider in Expression_Dark theme is most close to your requirements - see that image attached. What is left for you to modify is the Background of the RadSlider which you may set to #FF808080. For the BorderBrush and Background of the SelectionMiddleThumb to get modified, you should copy the default Slider style from your local installation folder (RadControls_for_Silverlight5_2013_1_0403_Trial\Themes\ExpressionDark\Themes\Expression\Dark\Charting) and do the modifications.
Then set the new modified style to the chart via our ChildrenOfType() extension method:
>().Style = this.Resources["slider"] as Style;
All the best,
the Telerik team