No vertical grid lines in graph?

[Uwe Dippel <udippel gmail com>]

At least, here I don't manage to get any. I have horizontal grid lines, major and minor. They also pop up at the 'guru'.

But when I 'add' to the 'X-axis', no grid option pops up. 

I guess it could have to make with the values of the x-axis: the twelve months of the year. And yet, the ticks show up, and I can select inside<->outside. So with the ticks showing up, I can see no reason not to be able to add grid lines. I agree, it is questionable to show vertical grid lines for a graph with the names of months as X-axis; and yet, I need my users being able to shift a ruler across the printout and read a value per month.

Any hint appreciated,

Uwe