statistics: implement a simple information box

When the user hovers over features in the chart, they should
be presented with more information. For example in bar charts
on the dives the bar represents and the exact value that the
bar represents, etc.

The InformationBox is a simple QGraphicsWidget, which can be
placed on top of QCharts and can show a number of arbitrary
text lines.

When placing the box on the chart, the code attempts to stay
inside the plot area of the chart.

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

stats/CMakeLists.txt

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

stats/informationbox.cpp

@@ -0,0 +1,75 @@
+#include "informationbox.h"
+#include "statscolors.h"
+#include "zvalues.h"
+
+#include <QChart>
+#include <QFontMetrics>
+
+static const QColor informationBorderColor(Qt::black);
+static const QColor informationColor(0xff, 0xff, 0x00, 192); // Note: fourth argument is opacity
+static const int informationBorder = 2;
+static const int distanceFromPointer = 10; // Distance to place box from mouse pointer or scatter item
+
+InformationBox::InformationBox(QtCharts::QChart *chart) : QGraphicsRectItem(chart), chart(chart)
+{
+	setPen(QPen(informationBorderColor, informationBorder));
+	setBrush(informationColor);
+	setZValue(ZValues::informationBox);
+}
+
+void InformationBox::setText(const std::vector<QString> &text, QPointF pos)
+{
+	width = height = 0.0;
+	textItems.clear();
+
+	for (const QString &s: text) {
+		if (!s.isEmpty())
+			addLine(s);
+	}
+
+	width += 4.0 * informationBorder;
+	height += 4.0 * informationBorder;
+
+	// Setting the position will also set the proper size
+	setPos(pos);
+}
+
+void InformationBox::setPos(QPointF pos)
+{
+	QRectF plotArea = chart->plotArea();
+
+	double x = pos.x() + distanceFromPointer;
+	if (x + width >= plotArea.right() && pos.x() - width >= plotArea.x())
+		x = pos.x() - width;
+	double y = pos.y() + distanceFromPointer;
+	if (y + height >= plotArea.bottom() && pos.y() - height >= plotArea.y())
+		y = pos.y() - height;
+
+	setRect(x, y, width, height);
+	double actY = y + 2.0 * informationBorder;
+	for (auto &item: textItems) {
+		item->setPos(QPointF(x + 2.0 * informationBorder, actY));
+		actY += item->boundingRect().height();
+	}
+}
+
+void InformationBox::addLine(const QString &s)
+{
+	textItems.emplace_back(new QGraphicsSimpleTextItem(s, this));
+	QGraphicsSimpleTextItem &item = *textItems.back();
+	item.setBrush(QBrush(darkLabelColor));
+	item.setPos(QPointF(0.0, height));
+	item.setFont(font);
+	item.setZValue(ZValues::informationBox);
+	QRectF rect = item.boundingRect();
+	width = std::max(width, rect.width());
+	height += rect.height();
+}
+
+// Try to stay within three-thirds of the chart height
+int InformationBox::recommendedMaxLines() const
+{
+	QFontMetrics fm(font);
+	int maxHeight = static_cast<int>(chart->plotArea().height());
+	return maxHeight * 2 / fm.height() / 3;
+}

stats/informationbox.h

@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: GPL-2.0
+// A small box displaying statistics information, notably
+// for a scatter-plot item or a bar in a bar chart.
+#ifndef INFORMATION_BOX_H
+#define INFORMATION_BOX_H
+
+#include <vector>
+#include <memory>
+#include <QGraphicsRectItem>
+#include <QFont>
+
+namespace QtCharts {
+	class QChart;
+}
+struct dive;
+
+// Information window showing data of highlighted dive
+struct InformationBox : QGraphicsRectItem {
+	InformationBox(QtCharts::QChart *chart);
+	void setText(const std::vector<QString> &text, QPointF pos);
+	void setPos(QPointF pos);
+	int recommendedMaxLines() const;
+private:
+	QtCharts::QChart *chart;
+	QFont font; // For future specialization.
+	double width, height;
+	void addLine(const QString &s);
+	std::vector<std::unique_ptr<QGraphicsSimpleTextItem>> textItems;
+};
+
+#endif