Y-axis values change depending on time span

Graphite does averaging.
Chances are that's what you're seeing.

-Nick

Henrik Møller Rasmussen <email address hidden> wrote:

>New question #199322 on Graphite:
>https://answers.launchpad.net/graphite/+question/199322
>
>Hi board,
>
>I've just started using StatsD and Graphite and having this strange issue.
>
>Depending on the time span, the Y-axis values change. See the two screenshot below.
>
>Correct: http://img.ly/j6WN
>Incorrect: http://img.ly/j6WU
>
>Is it a bug or have I misunderstood something?
>
>Thanks!
>
>/Henrik
>
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Hmm.. Why does it do averaging when I'm already using the summarize function? Is graphite not meant for these kind of long-spanning graphs?

During rendering, Graphite calculates the space between each point-draw to properly line up the values to the graph width. Once the space between the points gets below the line width (default 1.2 pixels) they start to 'smoosh' together, so at this point Graphite reduces the number of drawn points by consolidating them with an average() function. You can change this to a sum() function instead by wrapping the series in the cumulative() function (http://readthedocs.org/docs/graphite/en/0.9.10/functions.html#graphite.render.functions.cumulative) but note that it may not be what you expect as you'll be displaying the sum of the points which went into that drawn point (which may not line up with an actual value in the data).

If you're already using summarize, you can get the real values displayed by summarizing to a longer interval (thus reducing the number of points displayed). You can also fiddle with the threshold used before point consolidation occurs, minXStep: http://readthedocs.org/docs/graphite/en/0.9.10/render_api.html#minxstep and tweak the line width if the lines get too messy http://readthedocs.org/docs/graphite/en/0.9.10/render_api.html#linewidth

Also see this bug for some related discussion: https://bugs.launchpad.net/graphite/+bug/886411

Hmm.. Thanks for the answers guys. I'll try out the suggestions.

/Henrik