XAML

  <Line X1="10" X2="280" Y1="100" Y2="100" Stroke="Yellow" x:Name="MyLine" StrokeThickness="1" />

Rounded ends

  <Line X1="10" X2="280" Y1="100" Y2="100" Stroke="Yellow" x:Name="MyLine" StrokeThickness="6" StrokeStartLineCap="Round" StrokeEndLineCap="Round" />