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60 lines
2.9 KiB
Markdown
60 lines
2.9 KiB
Markdown
# Example (JKQTPlotter): Plotting Parametric Curves {#JKQTPlotterParametricCurves}
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This project (see `./examples/parametriccurve/`) demonstrates how to draw parametric curves `[x,y]=f(t)`, using `JKQTPXYLineGraph` and `JKQTPXYParametrizedScatterGraph`, i.e. from a set of coordinates `[xi,yi]`, for which the vector-values function `f(t)` is evaluated explicitly. If you are alloking for an example of implicit drawing by only defining the function `f(t)` and having JKQTPlotter evaluate it automatically and adaptively, you'll have to use `JKQTPXYFunctionLineGraph`, which is explained in [`./examples/evalcurve`](https://github.com/jkriege2/JKQtPlotter/tree/master/examples/evalcurve)
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The source code of the main application can be found in [`parametriccurve.cpp`](https://github.com/jkriege2/JKQtPlotter/tree/master/examples/parametriccurve/parametriccurve.cpp). First, the parametric curve (here a [logarithic spiral](https://en.wikipedia.org/wiki/Logarithmic_spiral)) is sampled into two columns containing the x- and y-values along the curve. In addition the radial distance from x=y=0 is added into a third column:
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```.cpp
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QVector<double> X, Y, R;
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const int Ndata=500; // number of plot points in each curve
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const double phiMax=4.0*M_PI;
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const double a=1;
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const double k=0.2;
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for (double phi=-phiMax; phi<=phiMax; phi+=phiMax/double(Ndata)) {
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const double x=a*exp(k*phi)*cos(phi);
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const double y=a*exp(k*phi)*sin(phi);
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X<<x;
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Y<<y;
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R<<sqrt(x*x+y*y);
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}
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// and copy it to the datastore
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size_t columnX=ds->addCopiedColumn(X, "x");
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size_t columnY=ds->addCopiedColumn(Y, "y");
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size_t columnR=ds->addCopiedColumn(R, "r");
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```
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Then simples graph just uses the columns X and Y to plot the curve:
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```.cpp
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JKQTPXYLineGraph* graph1=new JKQTPXYLineGraph(&plot);
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graph1->setXColumn(columnX);
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graph1->setYColumn(columnY);
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graph1->setDrawLine(true);
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graph1->setSymbolType(JKQTPNoSymbol);
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graph1->setTitle("one-colored spiral");
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plot.addGraph(graph1);
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```
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If you use `JKQTPXYParametrizedScatterGraph` instead of `JKQTPXYLineGraph`, you can also modify the color of the line-segments, connecting the datapoints:
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```.cpp
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JKQTPXYParametrizedScatterGraph* graph2=new JKQTPXYParametrizedScatterGraph(&plot2);
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graph2->setXColumn(columnX);
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graph2->setYColumn(columnY);
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graph2->setColorColumn(columnR);
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graph2->setColorPalette(ette(JKQTPMathImageMATLAB);
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graph2->setSymbolType(JKQTPNoSymbol);
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graph2->setDrawLine(true);
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graph2->setTitle("colored spiral");
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graph2->getColorBarRightAxis()->setAxisLabel("color scale radius $r(\\phi)$");
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plot2.addGraph(graph2);
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```
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The result looks like this:
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... and with the line-color set by the radius:
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