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c++qtgraphtrendlineqcustomplot

How to add a trendline to my graph in Qt?

I want to add a trendline to my graph in Qt.

How to plot a trendline on a line graph?

Solution

  • Here's how to compute a trendline mathematically:

    Slope:

    enter image description here

    Offset:

    enter image description here

    Trendline formula:

    enter image description here

    where

    a is slope
x is the horizontal axis value
b is the Y-intercept

    Normally you have sccattered data that makes up your graph. No matter if you use QtWidget or QML you can follow these steps.

    • First, find the average of the x values and the average of y values.

    • Next, find the distance between each x value and the average of the x values, and the distance between each y value and the average of the y values.

    • Then multiplying (x(i) - X(i)) by (y(i) - Y(i)) , and multiplying (x(i) - X(i)) by itself.

    • Get the sum of first formula ((x(i) - X(i)) by (y(i) - Y(i))) and sum of second formula ( (x(i) - X(i)) )

    • Find the slop a by dividing first formula sum by second formula sum.

    • Get the average value of x and y and find b value using trendline formula : y = ax + b so b = y - ax

    Then you have everything you want to apply your trendline equation

    Using your equation, you could now draw your line trend on your QCustomPLot using two point:

    QCPItemStraightLine *trendLine = new QCPItemStraightLine(customPlot);
trendLine->point1->setCoords(x, y);  // location of point 1 in plot coordinate
trendLine->point2->setCoords(xx, yy);  // location of point 2 in plot coordinate