I had the same problem and frustration (other than my error now says "Unable to load file" rather than "File unavailable", but that's probably just because it's been almost 2 years and they've changed their message). Solved for me by unchecking "Aggregate column A" on the Chart types tab of the Chart Editor.
Something else you may have to do if column A contains date-like info. In my spreadsheet, I had a 6-digit number representing
yyyymm ranging from
201506 to 201701 but the graph treated them as integers (as I suppose it should) jumping 89 spaces between 201512 and 201601. Checking that Aggregate column A box seemed to solve it immediately and gave me back my "months", but then I found later that adding trendlines was not working. If that's also your problem, you'll have to change date-like values to actual date values like
2017-01-01, and then just use custom number formatting to display them like
yyyymm or whatever you want.