QCustomPlot Discussion and Comments

I have a layout/margin question when having one QCustomPlot with multiple QCPAxisRect:s. I have tried to search for the answer in the support forum but not found the answer.

In my case the QCPAxisRect:s are stacked vertically and to save screen space I use no bottom axis label and the bottom axis tick labels are only enabled / visible for the bottom QCPAxisRect. This works fine but the height of the "graph area" of the last QCPAxisRect then gets a little bit smaller than the other. Whats the best way to automatically make sure that the "graph area" of all the QCPAxisRect:s have the same height?

I assume the issue is that the height of the tick labels (and also axis label if used) is added and thus decrease the available space for the rest.

I'm thinking about how to solve this:
1) Use axisrect->axis(QCPAxis::atBottom)->setTickLabelSide(QCPAxis::LabelSide::lsInside) - i.e. place the tick labels on the inside - this is what I currently do
2) Somehow manually add the tick labels as a separate layout item so that they are not part of the QCPAxisRect
3) Manually set margins or heights
4) ?

Here is an example - https://imgur.com/a/blVtMCK

Best regards
// Stefan

You could use an empty axisRect with no graph just to display the xAxis, and then add for each graph a new axisRect with only yAxis visible…

hello,BSO
When I use a blank axisRect to display the X-axis, how do I make it take up only the height of the X-axis and not the same height as the other axisRect with graph? (These Axisrects are added to customPlot via the addElement (row, clumn) function, arranged vertically.)

I found a workaround by setting the minimum and maximum dimensions of axisRect, and when the height under the maximum size is set to 1, the axisRect only X-axis looks no different from the other axisRect's own X-axis, which worked perfectly for what I wanted to achieve, thanks to BSO for the ideas! The code looks like the following, if you have a better solution or if there is any problem with my approach, please reply!

QCPAxisRect* channel = new QCPAxisRect(ui->customPlot, false);
channel->addAxes(QCPAxis::atBottom);
ui->customPlot->plotLayout()->addElement(channelNum, 0, channel);// Add to the bottom to act as the X-axis
// Modify the key two lines of code for sizing
channel->setMinimumSize(0,0);
channel->setMaximumSize(QWIDGETSIZE_MAX,1);

Thanks BSO. I used your suggestion and it works great. See https://imgur.com/a/JtSBfFI.

Good day!
stefanjohansss, Can you tell me about two vertical lines (with labels 'A' and 'B') and how you do that?

I've created a custom 'QCPCursor' class that inherit from QCPItemStraightLine. In this class I add a QCPItemText object and anchor it to the QCPItemPosition point1. Then it react to the mouseMoveEvent to set the coordinates and replot. To improve performance I only replot this layer (lmBuffered). This code can most likely be improved.

void Plot::build_cursor() {
    // Create separate layer to only replot this when moving the cursor
    qcpplot->addLayer("cursorLayer", 0, QCustomPlot::limAbove);
    qcpplot->layer("cursorLayer")->setMode(QCPLayer::lmBuffered);
    cursora = new QCPCursor(qcpplot, Qt::Orientation::Vertical, "A", 2);
    cursorb = new QCPCursor(qcpplot, Qt::Orientation::Vertical, "B", 4);
    qcpplot->replot();
}

qcpcursor.h

#ifndef QCPCURSOR_H
#define QCPCURSOR_H

#include "qcustomplot.h"
#include <QMouseEvent>
#include <QPointF>
#include <QVariant>
#include <QtCore>
#include <QtGui>

class QMouseEvent;
class QVariant;
class QPointF;

class QCPCursor : public QCPItemStraightLine {
    Q_OBJECT
  public:
    QCPCursor(QCustomPlot* parentPlot, Qt::Orientation orientation, QString name, double coordxy_initial);
    ~QCPCursor();
    Qt::Orientation orientation() const { return orientation_; }
    void setCoordsFromPosEmitAndReplot(QPointF const& pos);
    void setCoordsXAndReplot(double x);
    void setCoordsYAndReplot(double y);
    double coord_x;
    double coord_y;

  signals:
    void coordXChanged(double x);
    void coordYChanged(double y);

  protected:
    Qt::Orientation orientation_;
    QCPItemText* text;

  protected slots:
    void mousePressEvent(QMouseEvent* event, QVariant const& details);
    void mouseMoveEvent(QMouseEvent* event, QPointF const& startPos);
    void mouseReleaseEvent(QMouseEvent* event, QPointF const& startPos);
};

#endif // QCPCURSOR_H

qcpcursor.cpp

#include "qcpcursor.h"

QCPCursor::QCPCursor(QCustomPlot* parentPlot, Qt::Orientation orientation, QString name, double coordxy_initial)
    : QCPItemStraightLine(parentPlot) {

    this->orientation_ = orientation;

    this->setLayer("cursorLayer");
    this->setClipToAxisRect(false);

    QPen pen(Qt::lightGray);
    pen.setWidth(1); // Best performance with 1 width
    this->setPen(pen);

    // QCPItemText for cursor "name" (Normally A and B)
    text = new QCPItemText(parentPlot);
    text->setLayer("cursorLayer");
    text->setPositionAlignment(Qt::AlignHCenter | Qt::AlignCenter);
    text->setBrush(QBrush(Qt::lightGray));
    text->setColor(Qt::black);

    QFont font;
    font.setStyleHint(QFont::Helvetica);
    font.setBold(false);
    font.setPointSize(12);
    text->setFont(font);

    text->setText(name);
    
    text->position->setTypeX(QCPItemPosition::PositionType::ptAxisRectRatio);
    text->position->setTypeY(QCPItemPosition::PositionType::ptAxisRectRatio);
    text->position->setCoords(0, 0.05);
    text->position->setParentAnchorX(this->point1);

    if (orientation_ == Qt::Vertical) {
        coord_x = coordxy_initial;
        coord_y = 0;
        point1->setCoords(coord_x, 0);
        point2->setCoords(coord_x, 1);        
    } else {
        coord_x = 0;
        coord_y = coordxy_initial;
        point1->setCoords(0, coord_y);
        point2->setCoords(1, coord_y);
    }
}

QCPCursor::~QCPCursor() {
}

void QCPCursor::setCoordsFromPosEmitAndReplot(QPointF const& pos) {
    if (orientation() == Qt::Vertical) {
        coord_x = parentPlot()->xAxis->pixelToCoord(pos.x());
        coord_y = 0;
        point1->setCoords(coord_x, 0);
        point2->setCoords(coord_x, 1);

        // Emit that the coordination has changed
        emit(coordXChanged(coord_x));
    } else {
        coord_x = 0;
        coord_y = parentPlot()->yAxis->pixelToCoord(pos.y());
        point1->setCoords(0, coord_y);
        point2->setCoords(1, coord_y);

        // Emit that the coordination has changed
        emit(coordYChanged(coord_y));
    }

    // Only replot the layer with the cursor
    // https://www.qcustomplot.com/documentation/performanceimprovement.html
    parentPlot()->layer("cursorLayer")->replot();
}

void QCPCursor::setCoordsXAndReplot(double x) {
    if (orientation() == Qt::Vertical) {
        coord_x = x;
        coord_y = 0;
        point1->setCoords(coord_x, 0);
        point2->setCoords(coord_x, 1);

        // Do NOT emit when using this function (will cause infinite loop as this function is called to sync cursors for multiple plots when the signal coordXChanged is emitted)
        parentPlot()->layer("cursorLayer")->replot();
    }
}

void QCPCursor::setCoordsYAndReplot(double y) {
    if (orientation() == Qt::Horizontal) {
        coord_x = 0;
        coord_y = y;
        point1->setCoords(0, coord_y);
        point2->setCoords(1, coord_y);

        // Do NOT emit when using this function (will cause infinite loop as this function is called to sync cursors for multiple plots when the signal coordYChanged is emitted)
        parentPlot()->layer("cursorLayer")->replot();
    }
}

void QCPCursor::mousePressEvent(QMouseEvent* event, QVariant const& details) {
}

void QCPCursor::mouseMoveEvent(QMouseEvent* event, QPointF const& startPos) {
    setCoordsFromPosEmitAndReplot(event->pos());
}

void QCPCursor::mouseReleaseEvent(QMouseEvent* event, QPointF const& startPos) {
}