How to increase thickness of lines in Line Charts?

Click On chart, then you will see a Values filled and category groups Box on right side of graph, then in Values filled, right-click on any values Go to "Series Properties" --> "Border " -- > "Line Width" and then set the size.

chart1.Series["Default"].BorderWidth = 2;

Very helpful collection of examples: http://code.msdn.microsoft.com/Samples-Environments-for-b01e9c61