Grid Cross Plot: Add regression curves.

ApplicationLibCode/UserInterface/RiuQwtPlotRectAnnotation.h

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+/////////////////////////////////////////////////////////////////////////////////
+//
+//  Copyright (C) 2023-     Equinor ASA
+//
+//  ResInsight is free software: you can redistribute it and/or modify
+//  it under the terms of the GNU General Public License as published by
+//  the Free Software Foundation, either version 3 of the License, or
+//  (at your option) any later version.
+//
+//  ResInsight is distributed in the hope that it will be useful, but WITHOUT ANY
+//  WARRANTY; without even the implied warranty of MERCHANTABILITY or
+//  FITNESS FOR A PARTICULAR PURPOSE.
+//
+//  See the GNU General Public License at <http://www.gnu.org/licenses/gpl.html>
+//  for more details.
+//
+/////////////////////////////////////////////////////////////////////////////////
+
+#pragma once
+
+#include "qwt_interval.h"
+#include "qwt_plot_item.h"
+
+#include <QBrush>
+#include <QPen>
+
+#include <memory>
+
+class QRectF;
+class QPainter;
+class QwtPlotMarker;
+
+//==================================================================================================
+/// Rectangular plot item for annotation areas of a plot.
+///
+//==================================================================================================
+class RiuQwtPlotRectAnnotation : public QwtPlotItem
+{
+public:
+    RiuQwtPlotRectAnnotation();
+    ~RiuQwtPlotRectAnnotation() override = default;
+
+    void setInterval( double minX, double maxX, double minY, double maxY );
+
+    void        setPen( const QPen& );
+    const QPen& pen() const;
+
+    void          setBrush( const QBrush& );
+    const QBrush& brush() const;
+
+    void setText( const QString& text );
+
+    int    rtti() const override;
+    void   draw( QPainter* painter, const QwtScaleMap& xMap, const QwtScaleMap& yMap, const QRectF& canvasRect ) const override;
+    QRectF boundingRect() const override;
+
+    void attach( QwtPlot* plot );
+    void detach();
+    void setVisible( bool isVisible ) override;
+
+protected:
+    QwtInterval m_intervalX;
+    QwtInterval m_intervalY;
+    QPen        m_pen;
+    QBrush      m_brush;
+    QString     m_text;
+
+    std::unique_ptr<QwtPlotMarker> m_textLabel;
+};