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line chart standard deviations: how to change for subsets of data

I have a table with two fields (pH and time), and a new row is added to the table each day.

Using that table, I have created a line chart that has pH on the Y axis and time on the x axis.

I want to add lines to the chart that depict one standard deviation (SD). And (this is the part I don't understand), I want to recalculate the SD for a selected period of time, and then another for a different period of time.


In Year 1 the mean was 5 and the SD was 1. On the chart I want to plot pH vs time, plus the SD for year 1, which would be horizontal lines at pH 4 and pH 6.

Then, in Year 2, the mean was still 5, but the standard deviation was only 0.5. In the chart (which already shows the standard deviation for Year 1) I want to include that new SD, which would be horizontal lines at pH 4.5 and pH 5.5.

The end result would be a single chart that depicts pH vs time, plus two sets of SDs: one for Year 1 and another for Year 2.

Any advice?

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