Fix QXYSeries's best fit line not showing in some coordinate ranges

Use the bottom left point of the domain as a starting point.
Amends 28e2fb5a2891bcfb94cee288ec10909a1de29b10.

Fixes: QTBUG-108090
Task-number: QTBUG-89453
Change-Id: I28e948373106fa4d00ed132a6dd11afb7ef2ea6f
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>
Reviewed-by: Juha Vuolle <juha.vuolle@insta.fi>
(cherry picked from commit 2672db21a846c071cb31cfcfb974db5d837cfc42)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 9 insertions, 4 deletions

diff --git a/src/charts/xychart/qxyseries.cpp b/src/charts/xychart/qxyseries.cpp
index fc04e0f0..e01bf017 100644
--- a/src/charts/xychart/qxyseries.cpp
+++ b/src/charts/xychart/qxyseries.cpp
@@ -1907,13 +1907,18 @@ void QXYSeriesPrivate::drawBestFitLine(QPainter *painter, const QRectF &clipRect
     if (!ok)
         return;
 
-    const qreal x1 = clipRect.x();
+    auto *domain = this->domain();
+    const auto clipOriginX = domain->isReverseX() ? clipRect.right() : clipRect.left();
+    const auto clipOriginY = domain->isReverseY() ? clipRect.top() : clipRect.bottom();
+    const auto domainOrigin = domain->calculateDomainPoint({clipOriginX, clipOriginY});
+
+    const qreal x1 = domainOrigin.x();
     const qreal y1 = bestFitLineParams.first * x1 + bestFitLineParams.second;
-    QPointF p1 = domain()->calculateGeometryPoint(QPointF(x1, y1), ok);
+    QPointF p1 = domain->calculateGeometryPoint(QPointF(x1, y1), ok);
 
-    const qreal x2 = clipRect.x() + 1;
+    const qreal x2 = domainOrigin.x() + 1;
     const qreal y2 = bestFitLineParams.first * x2 + bestFitLineParams.second;
-    QPointF p2 = domain()->calculateGeometryPoint(QPointF(x2, y2), ok);
+    QPointF p2 = domain->calculateGeometryPoint(QPointF(x2, y2), ok);
 
     if (ok) {
         QLineF bestFitLine { p1, p2 };