How to move legend for 2D line chart

I have a 2D line chart with the X axis labels and Y axis colour legend (in an apparently default position) below the X axis line.

Ordinarily the labels ( if horizontal) and the legend would not interfere with each other - legend below the labels.

However, I need to have my labels on the diagonal, which means that the labels and the legend are now super-imposed over each other, meaning that neither label nor legend are easily legible.

But for the life of me I cannot see how to move the legend down below the labels, nor can I find a tutorial online.

Can anyone help, please?