I want to draw lines in the column header of a datagrid, using the following code. There are two ItemsControlwhich are both bound to a collection containing 18 Dates (first of every month). A converter calculates the position in the columnheader by the date, in order to place the controls in the columnheader.
The result is this: 
What I want, obviously, is to draw a red line over the black lines between the month names. It looks like that only every other line is drawn.
Has anyone an idea to solve this?