Adding a line to a bar graph

Evening guys,

(For this to make any sense please look at the attached sheet

I am trying to recreate the screen grab that I have copied and pasted in (purple bar chart). I am to create the bar chart, but I am unsure how to create the dotted line which is the national average. I would appreciate any help on this.

Many thanks

Hi Andy

When I try to add the error bars, they shift my column bar to the middle.
How do you format the x-error bars to draw the line - can you explain further?

Thanks