statistics: implement a legend box

For some chart (e.g. pie charts or stacked bar charts), we want
to display a legend. QtCharts' legend interface happens to be
private and therefore is of no use.

This introduces a legend box which is implemented using
QGraphicItems, which can be placed on top of QCharts. It's very
unfancy, but works for now. If there are too many items, not
all are shown. Currently, the legend is configured to fill
at most half of the width and half of the height of the chart.
This might need some optimization.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

stats/CMakeLists.txt

@@ -5,6 +5,8 @@ include_directories(.
 )
 
 set(SUBSURFACE_STATS_SRCS
+	legend.h
+	legend.cpp
 	statscolors.h
 	statscolors.cpp
 	statsvariables.h

stats/legend.cpp

@@ -0,0 +1,125 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "legend.h"
+#include "statscolors.h"
+#include <QFontMetrics>
+#include <QGraphicsSceneMouseEvent>
+#include <QGraphicsWidget>
+#include <QPen>
+
+static const double legendBorderSize = 2.0;
+static const double legendBoxBorderSize = 1.0;
+static const double legendBoxScale = 0.8;		// 1.0: text-height of the used font
+static const double legendInternalBorderSize = 2.0;
+static const QColor legendColor(0x00, 0x8e, 0xcc, 192); // Note: fourth argument is opacity
+static const QColor legendBorderColor(Qt::black);
+
+Legend::Legend(QGraphicsWidget *chart, const std::vector<QString> &names) :
+	QGraphicsRectItem(chart), chart(chart), displayedItems(0), width(0.0), height(0.0)
+{
+	setZValue(30.0); // On top of everything else.
+	entries.reserve(names.size());
+	int idx = 0;
+	for (const QString &name: names)
+		entries.emplace_back(name, idx++, (int)names.size(), this);
+
+	// Calculate the height and width of the elements
+	if (!entries.empty()) {
+		QFontMetrics fm(entries[0].text->font());
+		fontHeight = fm.height();
+		for (Entry &e: entries)
+			e.width = fontHeight + 2.0 * legendBoxBorderSize +
+				  fm.size(Qt::TextSingleLine, e.text->text()).width();
+	} else {
+		fontHeight = 0.0;
+	}
+	setPen(QPen(legendBorderColor, legendBorderSize));
+	setBrush(QBrush(legendColor));
+
+	resize(); // Draw initial legend
+}
+
+Legend::Entry::Entry(const QString &name, int idx, int numBins, QGraphicsItem *parent) :
+	rect(new QGraphicsRectItem(parent)),
+	text(new QGraphicsSimpleTextItem(name, parent))
+{
+	rect->setZValue(30.0);
+	rect->setPen(QPen(legendBorderColor, legendBoxBorderSize));
+	rect->setBrush(QBrush(binColor(idx, numBins)));
+	text->setZValue(30.0);
+	text->setBrush(QBrush(darkLabelColor));
+}
+
+void Legend::hide()
+{
+	for (Entry &e: entries) {
+		e.rect->hide();
+		e.text->hide();
+	}
+	QGraphicsRectItem::hide();
+}
+
+void Legend::resize()
+{
+	if (entries.empty())
+		return hide();
+
+	QSizeF size = chart->size();
+
+	// Silly heuristics: make the legend at most half as high and half as wide as the chart.
+	// Not sure if that makes sense - this might need some optimization.
+	int maxRows = static_cast<int>(size.height() / 2.0 - 2.0 * legendInternalBorderSize) / fontHeight;
+	if (maxRows <= 0)
+		return hide();
+	int numColumns = ((int)entries.size() - 1) / maxRows + 1;
+	int numRows = ((int)entries.size() - 1) / numColumns + 1;
+
+	double x = legendInternalBorderSize;
+	displayedItems = 0;
+	for (int col = 0; col < numColumns; ++col) {
+		double y = legendInternalBorderSize;
+		double nextX = x;
+
+		for (int row = 0; row < numRows; ++row) {
+			int idx = col * numRows + row;
+			if (idx >= (int)entries.size())
+				break;
+			entries[idx].pos = QPointF(x, y);
+			nextX = std::max(nextX, x + entries[idx].width);
+			y += fontHeight;
+			++displayedItems;
+		}
+		x += nextX;
+		width = nextX;
+		if (width >= size.width() / 2.0) // More than half the chart-width -> give up
+			break;
+	}
+	width += legendInternalBorderSize;
+	height = 2 * legendInternalBorderSize + numRows * fontHeight;
+	updatePosition();
+}
+
+void Legend::updatePosition()
+{
+	if (displayedItems <= 0)
+		return hide();
+	// For now, place the legend in the top right corner.
+	QPointF pos(chart->size().width() - width - 10.0, 10.0);
+	setRect(QRectF(pos, QSizeF(width, height)));
+	for (int i = 0; i < displayedItems; ++i) {
+		QPointF itemPos = pos + entries[i].pos;
+		QRectF rect(itemPos, QSizeF(fontHeight, fontHeight));
+		// Decrease box size by legendBoxScale factor
+		double delta = fontHeight * (1.0 - legendBoxScale) / 2.0;
+		rect = rect.adjusted(delta, delta, -delta, -delta);
+		entries[i].rect->setRect(rect);
+		itemPos.rx() += fontHeight + 2.0 * legendBoxBorderSize;
+		entries[i].text->setPos(itemPos);
+		entries[i].rect->show();
+		entries[i].text->show();
+	}
+	for (int i = displayedItems; i < (int)entries.size(); ++i) {
+		entries[i].rect->hide();
+		entries[i].text->hide();
+	}
+	show();
+}

stats/legend.h

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: GPL-2.0
+// A legend box, which is shown on the chart.
+#ifndef STATS_LEGEND_H
+#define STATS_LEGEND_H
+
+#include <memory>
+#include <vector>
+#include <QGraphicsRectItem>
+
+class QGraphicsSceneMouseEvent;
+
+class Legend : public QGraphicsRectItem {
+public:
+	Legend(QGraphicsWidget *chart, const std::vector<QString> &names);
+	void hover(QPointF pos);
+	void resize(); // called when the chart size changes.
+private:
+	// Each entry is a text besides a rectangle showing the color
+	struct Entry {
+		std::unique_ptr<QGraphicsRectItem> rect;
+		std::unique_ptr<QGraphicsSimpleTextItem> text;
+		QPointF pos;
+		double width;
+		Entry(const QString &name, int idx, int numBins, QGraphicsItem *parent);
+	};
+	QGraphicsWidget *chart;
+	int displayedItems;
+	double width;
+	double height;
+	int fontHeight;
+	std::vector<Entry> entries;
+	void updatePosition();
+	void hide();
+};
+
+#endif