How to plot a constant value in a line chart

Hello All, 

I have a line chart that displays how much electricty was used at each location for 5 days. The table has:  Location, Date (5days), Time (each hour) and Value. I have plotted the visual with location,date, time in the x axis and Value in the y axis, filtered by location which works as expected.

I have a different table that has the legal limit value for each location that I want to add to the visual to show the outputs vs the limit. However, at the moment when I add the 'legal limit' value into the y axis , Im getting a total count of all the values and not the singular/constant value for each location? (showing as a straight line over time) 

I have checked that the tables's relashionship is activated so I cant figure why it wont plot the singular value. Ive changed the data type too but no luck.

Any suggestions would be great, thank you.