QtCharts: how to plot line series with error bars - c++

Question

I have seen this question already being asked:
in Qt forum topic
in another Qt forum topic
in yet another Qt forum topic
and even in SO
Surprisingly I found no solutions (well, qcustomplot has this feature, but we are not allowed to use it in my company). My task is to represent some scientific data as a line plot with error bars representing the data standard deviation. I have tried some hacks to solve it, but the results are not sufficient for our application:
creating one QLineSeries for data mean values, and then add N QLineSeries to represent error bars (see approachWithMultipleQLineSeries in the example below, ); this is clearly an overkill and my plot is glitching on the slower machines (with Windows :/)
creating one QLineSeries for data mean values and then add QBoxPlotSeries to represent error bars (see approachWithQBoxSet in the example below, ); this does not glitch anymore, but the error bars do not correspond well to the line plot (e.g. look at the values close to zero), probably because Boxes are designed for slightly different X axes (with categories, not floating point numbers);
Could you please suggest some better approach where error bar values will actually be associated with X value and mean Y value (maybe subclass QXYSeries, or sth similar, I do not have any good ideas how to do it exactly)?
MCVE if someone would like to try:
CMakeLists.txt:
cmake_minimum_required(VERSION 3.11.0)
project(QtChartsErrBars CXX)
set(CMAKE_CXX_STANDARD 17)
find_package(Qt5 COMPONENTS Charts Core Widgets REQUIRED)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)
set(CMAKE_AUTOUIC ON)
add_executable(QtChartsErrBars
main.cpp
Demo.cpp
Demo.hpp
)
target_link_libraries(QtChartsErrBars
Qt5::Charts
Qt5::Core
Qt5::Widgets
)
target_compile_options(QtChartsErrBars PUBLIC -g)
Demo.hpp:
#pragma once
/**
* /author Pawel Ptasznik
*/
#include <QMainWindow>
#include <QWidget>
#include <memory>
class Demo : public QMainWindow
{
Q_OBJECT
public:
explicit Demo(QWidget* parent = nullptr);
~Demo() override = default;
};
Demo.cpp:
#include "Demo.hpp"
#include <QDebug>
#include <QVector>
#include <QtCharts/QChartView>
#include <QtCharts/QLineSeries>
#include <QtCharts/QAreaSeries>
#include <QtCharts/QLegendMarker>
#include <QtCharts/QBoxSet>
#include <QtCharts/QBoxPlotSeries>
#include <algorithm>
#include <random>
static std::random_device rd{};
static std::mt19937 gen{ rd() };
// for each input generate n samples using normal distribution with expected value == x^2
QVector<QVector<double> > generateData(QVector<double> in, int nSamples)
{
QVector<QVector<double> > results(in.size());
for (int i = 0; i < in.size(); i++)
{
results[i].resize(nSamples);
std::normal_distribution<double> d{ in[i]*in[i], 1 };
for (auto& sample : results[i])
{
sample = d(gen);
}
}
return results;
}
/************* HERE the solutions I have tried and I do not like ************/
namespace
{
class VerticalLine : public QtCharts::QLineSeries
{
public:
VerticalLine(double x, double yMin, double yMax)
{
replace(QVector<QPointF>{ { x, yMin }, { x, yMax } });
}
};
}
void approachWithMultipleQLineSeries(QtCharts::QChart *chart,
const QVector<double>& x, const QVector<double>& yMean, const QVector<double>& yStdDev)
{
for (int i = 0; i < x.size(); i++)
{
auto series = new VerticalLine(x[i], yMean[i]-yStdDev[i], yMean[i]+yStdDev[i]);
chart->addSeries(series);
chart->legend()->markers(series)[0]->setVisible(false);
}
}
void approachWithQBoxSet(QtCharts::QChart *chart,
const QVector<double>& x, const QVector<double>& yMean, const QVector<double>& yStdDev)
{
QList<QtCharts::QBoxSet*> boxList;
for (int i = 0; i < x.size(); i++)
{
boxList.push_back(new QtCharts::QBoxSet(yMean[i]-yStdDev[i], yMean[i]-yStdDev[i]/2,
yMean[i], yMean[i]+yStdDev[i]/2, yMean[i]+yStdDev[i]));
}
auto series = new QtCharts::QBoxPlotSeries();
series->append(boxList);
series->setBoxWidth(0.1);
series->attachAxis(chart->axes(Qt::Horizontal)[0]);
chart->addSeries(series);
chart->legend()->markers(series)[0]->setVisible(false);
}
/************* HERE quick example that plots quadratic function over 200 points ************/
Demo::Demo(QWidget* parent): QMainWindow(parent)
{
QVector<double> xAxisValues(200); // lets have 200 points between -10 and 10
std::generate(xAxisValues.begin(), xAxisValues.end(), [x = -10.0]() mutable { return x += 0.1; });
QVector<double> yAxisMeanValues(xAxisValues.size());
QVector<double> yAxisStdDev(xAxisValues.size());
// lets generate 50 samples for each x value
auto data = generateData(xAxisValues, 50);
// calculating mean values
std::transform(data.begin(), data.end(), yAxisMeanValues.begin(), [](const auto& samples)
{
double sum = 0.0;
for (auto sample : samples) sum += sample;
return sum / samples.size();
});
// calculating standard deviation
std::transform(data.begin(), data.end(), yAxisMeanValues.begin(), yAxisStdDev.begin(), [](const auto& samples, auto mean)
{
double var = 0.0;
for (auto sample : samples) var += (sample - mean)*(sample - mean);
var /= samples.size();
return std::sqrt(var);
});
QtCharts::QLineSeries *series = new QtCharts::QLineSeries();
for (int i = 0; i < xAxisValues.size(); i++)
{
series->append(xAxisValues[i], yAxisMeanValues[i]);
}
QtCharts::QChart *chart = new QtCharts::QChart();
chart->addSeries(series);
// OK, now how do I add the error bars basing on x, yMean, yStdDev?
// approachWithMultipleQLineSeries(chart, xAxisValues, yAxisMeanValues, yAxisStdDev);
chart->createDefaultAxes();
approachWithQBoxSet(chart, xAxisValues, yAxisMeanValues, yAxisStdDev);
chart->setTitle("Demo chart");
QtCharts::QChartView *chartView = new QtCharts::QChartView(chart);
chartView->setRenderHint(QPainter::Antialiasing);
setCentralWidget(chartView);
resize(400, 300);
}
main.cpp:
#include "Demo.hpp"
#include <QApplication>
int main(int argc, char** argv)
{
QApplication a(argc, argv);
Demo mainWindow;
mainWindow.show();
return a.exec();
}