FlowingData Forums » Data Visualization

How to compare line graphs of metrics that have different units.

Started 1 year ago by tomr / 3 posts

  1. tomr
    Member
    Posted 1 year ago #

    What is the best approach for comparing data series that have completely different units?

    For example the user may wish to look at a report to compare CPU (%) Disk Space (GB), Disk I/O (Read/s) and Paging (pages / second) to determine if there's any correlation.

    Or they might know there's a problem but need to see a graph to work out which particular metrics may have caused it.

    A common approach is to plot all the lines on the same y-axis. This renders the scale slightly useless but does allow for easy comparison.

    The other approach is to have seperate graphs for each metric. This is allows y axises to show units and avoids arbitary scaling, but is harder to spot trends between series.

    Are there other solutions? What's the best approach?

  2. nathany
    FlowingData Editor
    Posted 1 year ago #

    x-y scatterplot?

  3. EllieK
    Member
    Posted 1 year ago #

    If you are willing to use only two dependent variables per chart, you could try comparisons using the same x-axis with two y-axes. One would be on the left in the usual spot, and the y-axis for the other metric would be on the right.

    That way you could use the correct units for each metric as it would be on it's own axis. Scaling and even which metrics to use for each graph would be a judgement call and might not be suitable for your data at all. But I've done this in the past, usually with both y-axes being base 10, tho I guess you could use log 2 or other scales if appropriate. Sorry, I can't remember what these graphs are called, but MS Excel gives them a descriptive name with examples. So does SPSS for PC.


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