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How do I enter time-only data and analyze time series data

Whenever you enter only times into a data column, SigmaPlot by default uses the system zero date of 4713 BC. This is intentional, because any modern date creates such large internal numbers that any sort of statistical analysis is extremely difficult if not impossible.

The problem begins to occur if you reach a point where you move to the next day. This is not an issue if you are using a cyclical axis, i.e., a polar plot. However, if you are using a linear, Cartesian axis, you will need to enter an actual date as well as a time for the time data to continue rather than restart on the same day.

The key to understanding SigmaPlot’s date and time data is that it all occurs on a continuous, historical axis. Therefore, if you enter only times, they will always fall on Jan 1, 4713 BC. If you want to enter a time for the next day, you need to set the date to 1/2/4713.

For example, the following data are entered :

The expectation is that this data will be a continuous line, but instead it plots as:

What is going on here? The problem is that the second set of data is occurring on the same day as the first set, as we can see if we display the full day and month for the data using the Format Cells dialog box:

With the month and day also showing,

If we simply change the times to include the next day:

Then we get:

 
 
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