I'm developing a Windows Phone 8.1 app and I have to draw a chart (line chart). I know there is the WinRT XAML Toolkit but it seems not working for WP8.1 but only for Windows 8.1 apps, like many others libraries.
There is a stable library for Windows Phone 8.1 too?
Telerik's RadCharts are quite good
http://www.telerik.com/products/windows-phone/overview/all-controls/chart.aspx
WinRT XAML Toolkit is only for Apps which runs on WinRT (Store Apps).
To draw charts in Windows Phone you can use on of the following libraries:
1. AmCharts
2. QuickCharts
3. Sparrow Toolkit
You can install them in your project through NuGet.