tdf#133423 Fix 'central' formula for moving average

Change-Id: I3685c310fdbe4809cb7863628be96645364c6a7a
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/112191
Tested-by: Jenkins
Reviewed-by: Samuel Mehrbrodt <samuel.mehrbrodt@allotropia.de>

2 files changed, 37 insertions, 12 deletions

diff --git a/chart2/source/inc/MovingAverageRegressionCurveCalculator.hxx b/chart2/source/inc/MovingAverageRegressionCurveCalculator.hxx
index 8ba5456c2939..8bc76a33762e 100644
--- a/chart2/source/inc/MovingAverageRegressionCurveCalculator.hxx
+++ b/chart2/source/inc/MovingAverageRegressionCurveCalculator.hxx
@@ -52,9 +52,8 @@ private:
         const css::uno::Reference<css::chart2::XScaling>& xScalingY,
         sal_Bool bMaySkipPointsInCalculation ) override;
 
-    void calculateValues(RegressionCalculationHelper::tDoubleVectorPair aValues, sal_Int32 nPeriod,
-                         bool bUseXAvg);
-
+    void calculateValues(RegressionCalculationHelper::tDoubleVectorPair aValues, bool bUseXAvg);
+    void calculateValuesCentral(RegressionCalculationHelper::tDoubleVectorPair aValues);
     std::vector<double> aYList;
     std::vector<double> aXList;
 };
diff --git a/chart2/source/tools/MovingAverageRegressionCurveCalculator.cxx b/chart2/source/tools/MovingAverageRegressionCurveCalculator.cxx
index bca154047935..5496f78577e6 100644
--- a/chart2/source/tools/MovingAverageRegressionCurveCalculator.cxx
+++ b/chart2/source/tools/MovingAverageRegressionCurveCalculator.cxx
@@ -53,45 +53,71 @@ void SAL_CALL MovingAverageRegressionCurveCalculator::recalculateRegression(
     aYList.clear();
     aXList.clear();
 
+    // For formulas, see
+    // https://docs.oasis-open.org/office/OpenDocument/v1.3/cs02/part3-schema/OpenDocument-v1.3-cs02-part3-schema.html#property-chart_regression-moving-type
+
     switch (mnMovingType)
     {
         case MovingAverageType::Central:
         {
-            sal_Int32 nCentralPeriod = (mPeriod % 2 == 0) ? (mPeriod / 2) : ((mPeriod - 1) / 2);
-            calculateValues(aValues, nCentralPeriod, false);
+
+            calculateValuesCentral(aValues);
             break;
         }
 
         case MovingAverageType::AveragedAbscissa:
         {
-            calculateValues(aValues, mPeriod, true);
+            calculateValues(aValues, true);
             break;
         }
         case MovingAverageType::Prior:
         default:
         {
-            calculateValues(aValues, mPeriod, false);
+            calculateValues(aValues, false);
             break;
         }
     }
 }
 
+void MovingAverageRegressionCurveCalculator::calculateValuesCentral(
+    RegressionCalculationHelper::tDoubleVectorPair aValues)
+{
+    const size_t aSize = aValues.first.size();
+    for (size_t i = mPeriod - 1; i < aSize; ++i)
+    {
+        double yAvg = 0.0;
+
+        for (sal_Int32 j = 0; j < mPeriod; j++)
+        {
+            yAvg += aValues.second[i - j];
+        }
+        yAvg /= mPeriod;
+        aYList.push_back(yAvg);
+    }
+    sal_Int32 nPeriodLocal = (mPeriod % 2 == 0) ? (mPeriod / 2) : ((mPeriod - 1) / 2);
+    for (size_t i = nPeriodLocal; i < aSize - 1; ++i)
+    {
+        double x = aValues.first[i];
+        aXList.push_back(x);
+    }
+}
+
 void MovingAverageRegressionCurveCalculator::calculateValues(
-    RegressionCalculationHelper::tDoubleVectorPair aValues, sal_Int32 nPeriod, bool bUseXAvg)
+    RegressionCalculationHelper::tDoubleVectorPair aValues, bool bUseXAvg)
 {
     const size_t aSize = aValues.first.size();
-    for (size_t i = nPeriod - 1; i < aSize; ++i)
+    for (size_t i = mPeriod - 1; i < aSize; ++i)
     {
         double xAvg = 0.0;
         double yAvg = 0.0;
 
-        for (sal_Int32 j = 0; j < nPeriod; j++)
+        for (sal_Int32 j = 0; j < mPeriod; j++)
         {
             xAvg += aValues.first[i - j];
             yAvg += aValues.second[i - j];
         }
-        yAvg /= nPeriod;
-        xAvg /= nPeriod;
+        yAvg /= mPeriod;
+        xAvg /= mPeriod;
 
         aYList.push_back(yAvg);
         if (bUseXAvg)