Adding reference lines to a chart with primary and secondary axis

Good day all! New to the forum! I have a chart with sales data on one axis and inventory on a secondary axis. I have the average of each columns of data and want to add a reference line to each axis with it's average. I have added the averages to my data set, but it only adds the reference line to my primary axis. I actually began by manually inserting two lines to the chart (which seems asinine). They actually didn't print because they weren't actually on the chart, but rather just on the spreadsheet behind. So neither solution is working for me right now. Please help, thanks!

Ben

So.....how do I attach a file to a reply? I can't find anywhere to do that on here!

Great! Thank you Andy!!