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Thread: QWT and time series

  1. #1

    Default QWT and time series

    Hi guys!

    Im currently trying to use QWT in my application, which is hugely using time series data. I have re-implemented QwtScaleDraw to put dates on x-axis, yet i have problems with graph itself. Problem is that the time points requested by QWT during axis drawing are nowhere near days/hours boundaries and seem fractional , so i can not put time labels because they represent nonexisting points in data( which is hourly ).
    Is there any way I can force QWT graph to use day and hour boundaries, like to put major axis labels straight on date 0:00 am , or select axis points among fixed set of possible values ?

    I also have some related questions :
    - how do i put overlay bar under graph to mark for example selected timespan ( ie day ) ? ( Assuming I have data range and I want to apply it to graph object )
    - if I provide a set of ~1000 data points to plot it become slow to rescale. Is there something I missed in plot parameters or it is normal qwt & desktop computer to work like this with datasets of this size? If latter is actual, should i do some data preprocessing to reduce ammount of points or there is some existing soluton, maybe inside qwt?

    Thanks in advance!

    PS: regarding rescale , i found that it is not dependent on data point count, but related to how it works with splitter adjacent to plot itself. I've attached sample code which shows a problem.
    slow_plot.zip

    Is it a known issue? does any workaround exist?
    Last edited by Neekeetos; 6th February 2012 at 10:21. Reason: additional data on mentioned

  2. #2
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    Default Re: QWT and time series

    Quote Originally Posted by Neekeetos View Post
    Is there any way I can force QWT graph to use day and hour boundaries, like to put major axis labels straight on date 0:00 am , or select axis points among fixed set of possible values ?
    Qwt doesn't have a scale engine for time/date values - it's on my TODO list for the next release of Qwt. So for the moment you have to implement your own scale engine. "All" you need to do is to implement the pure virtual methods of QwtScaleEngine.

    But if you have a static situation ( no zooming, known series ) you don't need to have a scale engine because you could assign all ticks manually by QwtPlot::setAxisScaleDiv().

    Uwe

  3. #3

    Default Re: QWT and time series

    Quote Originally Posted by Uwe View Post
    Qwt doesn't have a scale engine for time/date values - it's on my TODO list for the next release of Qwt. So for the moment you have to implement your own scale engine. "All" you need to do is to implement the pure virtual methods of QwtScaleEngine.
    Uwe, thanks for reply! I will re-implement scale engine as you said. There is another side question I encounterd - how do I provide date/time information to plot/scaleengine without converting it to other data types like int/unixtime ? The only way to assign time series data to plot I found is to mimic data to PointF which imply, that inside plot classes, I need to know exact format to convert X back to date. Maybe there is something I'm missing?

    Thanks in advance!

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    Default Re: QWT and time series

    Hey Uwe we just wanted to know if you have implemented the time series scale engine.
    Thanks a lot

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    Default Re: QWT and time series

    Quote Originally Posted by Udayagiri Rithwik View Post
    Hey Uwe we just wanted to know if you have implemented the time series scale engine.
    http://qwt.sourceforge.net/class_qwt...le_engine.html

    Uwe

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    Smile Re: QWT and time series

    Thank you Uwe

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