How to remove the x-axis rescale motion when a tick label appears in a realtime plot?

[Phataas]

I have a real time plot where data is continuously added. When a tick label appears from the right of the x-axis (time) the axis is compressed a little and when the tick label disappears on the left side of the x-axis it is stretched a little. This affects my waveforms and also looks like lag.

The only scaling I am doing in the initialization of the plot is:

qwtDateScaleDraw = new QwtDateScaleDraw (Qt::OffsetFromUTC);
    qwtDateScaleDraw->setDateFormat(QwtDate::Second, "hh:mm:ss");
    qwtDateScaleDraw->setUtcOffset(QDateTime::currentMSecsSinceEpoch() / 1000);

qwtDateScaleDraw = new QwtDateScaleDraw (Qt::OffsetFromUTC);
    qwtDateScaleDraw->setDateFormat(QwtDate::Second, "hh:mm:ss");
    qwtDateScaleDraw->setUtcOffset(QDateTime::currentMSecsSinceEpoch() / 1000);

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And when new data is appended to the plot I rescale it to always have the previous 10 seconds of data visible by doing this:

plot->setAxisScale(QwtPlot::xBottom, timeStamp - 10000, timeStamp, 1000);

plot->setAxisScale(QwtPlot::xBottom, timeStamp - 10000, timeStamp, 1000);

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(where timeStamp is QDateTime::currentMSecsSinceEpoch())

I have been trying to get this to work for days now, but I can not figure it out =/

[Uwe]

This has to do with the rightmost tick label, that needs to have enough space.

The most natural way to avoid this problem is to use the space below the left y axis. F.e. in the cpuplot example the tick labels are rotated and shifted, so that there is no overlapping part to the right. Another trick is simply to add an axis to the right ( having the same scale as the left one ).

Next option is to overload QwtScaleDraw::tickLabel(), where you could decide to return QString::null when the most right tick is too close to the right border. "Too close" in this context means something in terms of 0.5 * QFontMetrics::width( label ) and the distance between the tick position and the border of the canvas in pixels.

I also has a quick check, what needs to be done, when accepting, that the labels can be cut off. Would make sense for your use case, but I need more time on this and I'm afraid that won't be possible with Qwt 6.1 without patching some code.

Uwe