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c++qtqwt

recalculate QwtScaleDiv before rendering

I've implemented a QwtPlot which scrolls across the screen as data is added in real-time. Based on user input, an image of the plot is occasionally rendered to a file using QwtPlotRenderer. However, because the axis scrolls during normal operation, the QwtScaleDiv tick marks can look a little wonky at render time (they are right-aligned):

enter image description here

Is there some easy way in which I can recalculate the division prior to rendering so that the first label is on the far left and the last one is on the far right?

Solution

  • This isn't as difficult as it looked at first. Bascially, all you need to do is temporarily replace the axisScaleDiv.

    auto divX = this->axisScaleDiv(xBottom);

double ub = divX.upperBound();
double lb = divX.lowerBound();
double numTicks = 11.0;  // 10 even divisions

// you can create minor/medium ticks if you want to, I didn't.
QList<double> majorTicks;
for (int i = 0; i < numTicks; ++i)
{
    majorTicks.push_back(lb + i * ((ub - lb) / (numTicks - 1)));
}

// set the scale to the newly created division
QwtScaleDiv renderDivX(divX.lowerBound(), divX.upperBound(), 
    QList<double>(), QList<double>(), majorTicks);  
this->setAxisScaleDiv(xBottom, renderDivX);

// DO PLOT RENDERING
QwtPlotRender renderer;
renderer.renderDocument(...);

// RESOTRE PREVIOUS STATE
this->setAxisScaleDiv(xBottom, divX);
this->setAxisScaleDiv(yLeft, divY);

// update the axes
this->updateAxes();