Line graph to show Reading Ages

I find that a lot of charting questions end up being more spreadsheet questions. Once the spreadsheet issues are resolved, creating the chart is often easy. In your case, I see to basic spreadsheet issues that need to be addressed before we can talk about creating a chart.

1) Excel expects to see data for a chart arranged in a certain way, and what you are showing here is not very conducive to a chart. I can only hope that you are allowed to rearrange the data (or a copy of the data) into something much more conducive to creating a chart. This essay covers how to arrange data in the sheet to make it easiest to create charts: https://peltiertech.com/good-chart-data/

2) The other issue (and this may cause you to rethink the solutions provided in your previous about how to "calculate" these values) is that all of your desired chart data is text. 10:12 cannot be a number (unless it is a time number, in which case the underlying value is completely wrong for your application). How critical is this yrs:mnth notation (with years and months separated by a colon ":")? As a final, "dead end" sort of thing, these text strings are okay. But, for an intermediate step where the values will be subject to further analysis and charting, text is difficult to useless. If the notation is not essential, I would store these as simple decimal years (10 11/12, for example. or enter 10.11 and use the DOLLARDE() function to convert to decimal years). If the notation is essential, then I would still store the real numbers as decimal years, and maintain a "parallel" block of cells that converts the numbers to yrs:mnths text.

Assuming those changes are allowed, I would expect to build the chart by:

1) putting "date of test" in the leftmost column of my chart range.
2) adding as many columns of "reading age" as needed with the reading ages stored as numeric decimal year values.
3) Select this block of cells and insert the desired chart.
4) Check that Excel correctly parsed the different chart elements (as Peltier explains, if you pay careful attention to steps 1 and 2, then Excel will usually get this correct).
5) Format chart elements as desired.

As a broad overview, that's what I would expect. I am uncertain about what you can and cannot do with your data, so I haven't taken the time to piece together details. Let us know more about your requirements and, assuming the steps I've outlined are allowed, what steps you have trouble with.