add an optional goal and linear regression weight line

android_app/app/src/main/java/com/health/openscale/gui/fragments/GraphFragment.java

@@ -20,6 +20,7 @@ import android.content.Intent;
 import android.content.SharedPreferences;
 import android.content.res.ColorStateList;
 import android.graphics.Color;
+import android.graphics.DashPathEffect;
 import android.os.Bundle;
 import android.preference.PreferenceManager;
 import android.support.design.widget.FloatingActionButton;
@@ -332,6 +333,137 @@ public class GraphFragment extends Fragment implements FragmentUpdateListener {
             enableMonth.setBackgroundTintList(ColorStateList.valueOf(Color.parseColor("#d3d3d3")));
         }
 
+        if (prefs.getBoolean("goalLine", true)) {
+            Stack<PointValue> valuesGoalLine = new Stack<PointValue>();
+
+            float goalWeight = openScale.getSelectedScaleUser().goal_weight;
+
+            valuesGoalLine.push(new PointValue(0, goalWeight));
+            valuesGoalLine.push(new PointValue(31, goalWeight));
+
+            Line goalLine = new Line(valuesGoalLine)
+                    .setHasPoints(false);
+
+            goalLine.setPathEffect(new DashPathEffect(new float[] {10,30}, 0));
+
+            lines.add(goalLine);
+        }
+
+        if (prefs.getBoolean("regressionLine", true)) {
+
+            /*
+            // quadratic regression y = ax^2 + bx + c
+            double x_value = 0.0;
+            double y_value = 0.0;
+
+            double s40 = 0; //sum of x^4
+            double s30 = 0; //sum of x^3
+            double s20 = 0; //sum of x^2
+            double s10 = 0;  //sum of x
+            double s00 = scaleDataList.size();
+            //sum of x^0 * y^0  ie 1 * number of entries
+
+            double s21 = 0; //sum of x^2*y
+            double s11 = 0;  //sum of x*y
+            double s01 = 0;   //sum of y
+
+            for(ScaleData scaleEntry: scaleDataList) {
+                calDB.setTime(scaleEntry.getDateTime());
+
+                x_value = calDB.get(field);
+                y_value = scaleEntry.getConvertedWeight(openScale.getSelectedScaleUser().scale_unit);
+
+                s40 += Math.pow(x_value, 4);
+                s30 += Math.pow(x_value, 3);
+                s20 += Math.pow(x_value, 2);
+                s10 += x_value;
+
+                s21 += Math.pow(x_value, 2) * y_value;
+                s11 += x_value * y_value;
+                s01 += y_value;
+            }
+
+            // solve equations using Cramer's law
+            double a = (s21*(s20 * s00 - s10 * s10) -
+                        s11*(s30 * s00 - s10 * s20) +
+                        s01*(s30 * s10 - s20 * s20))
+                        /
+                        (s40*(s20 * s00 - s10 * s10) -
+                        s30*(s30 * s00 - s10 * s20) +
+                        s20*(s30 * s10 - s20 * s20));
+
+            double b = (s40*(s11 * s00 - s01 * s10) -
+                        s30*(s21 * s00 - s01 * s20) +
+                        s20*(s21 * s10 - s11 * s20))
+                        /
+                        (s40 * (s20 * s00 - s10 * s10) -
+                        s30 * (s30 * s00 - s10 * s20) +
+                        s20 * (s30 * s10 - s20 * s20));
+
+            double c = (s40*(s20 * s01 - s10 * s11) -
+                        s30*(s30 * s01 - s10 * s21) +
+                        s20*(s30 * s11 - s20 * s21))
+                        /
+                        (s40 * (s20 * s00 - s10 * s10) -
+                        s30 * (s30 * s00 - s10 * s20) +
+                        s20 * (s30 * s10 - s20 * s20));
+            */
+
+            // linear regression y = a + x*b
+            double sumx = 0.0;
+            double sumy = 0.0;
+
+            double x_value = 0.0;
+            double y_value = 0.0;
+
+            for(ScaleData scaleEntry: scaleDataList) {
+                calDB.setTime(scaleEntry.getDateTime());
+
+                x_value = calDB.get(field);
+                y_value = scaleEntry.getConvertedWeight(openScale.getSelectedScaleUser().scale_unit);
+
+                sumx += x_value;
+                sumy += y_value;
+            }
+
+            double xbar = sumx / scaleDataList.size();
+            double ybar = sumy / scaleDataList.size();
+
+            double xxbar = 0.0;
+            double xybar = 0.0;
+
+            for(ScaleData scaleEntry: scaleDataList) {
+                calDB.setTime(scaleEntry.getDateTime());
+
+                x_value = calDB.get(field);
+                y_value = scaleEntry.getConvertedWeight(openScale.getSelectedScaleUser().scale_unit);
+
+                xxbar += (x_value - xbar) * (x_value - xbar);
+                xybar += (y_value - xbar) * (y_value - ybar);
+            }
+
+            double b = xybar / xxbar;
+            double a = ybar - b * xbar;
+
+
+            Stack<PointValue> valuesLinearRegression = new Stack<PointValue>();
+
+            for (int i = 0; i < 31; i++) {
+                y_value = a + b * i; // linear regression
+                //y_value = a * i*i + b * i + c; // quadratic regression
+
+                valuesLinearRegression.push(new PointValue((float) i, (float) y_value));
+            }
+
+            Line linearRegressionLine = new Line(valuesLinearRegression)
+                    .setColor(ChartUtils.COLOR_VIOLET)
+                    .setHasPoints(false);
+
+            linearRegressionLine.setPathEffect(new DashPathEffect(new float[] {10,30}, 0));
+
+            lines.add(linearRegressionLine);
+        }
+
         LineChartData lineData = new LineChartData(lines);
         lineData.setAxisXBottom(new Axis(axisValues).
                 setHasLines(true).

android_app/app/src/main/res/values/strings.xml

@@ -150,6 +150,9 @@
     <string name="label_ignoreOutOfRange">Ignore data that are out of range</string>
     <string name="label_initial_weight">Initial weight</string>
     <string name="label_average_data">Calculate average per day/month</string>
+    <string name="label_regression_line">Regression weight line</string>
+    <string name="label_goal_line">Goal line</string>
+
 
     <string name="error_max_scale_users">Maximum number of concurrent scale users reached.</string>
     <string name="info_step_on_scale">Please step barefoot on the scale for reference measurements.</string>

android_app/app/src/main/res/xml/graph_preferences.xml

@@ -4,4 +4,5 @@
     <CheckBoxPreference android:title="@string/label_enable_points" android:summaryOn="@string/info_is_visible" android:summaryOff="@string/info_is_not_visible" android:key="pointsEnable" android:defaultValue="true"/>
     <CheckBoxPreference android:title="@string/label_delete_confirmation" android:summaryOn="@string/info_is_enable" android:summaryOff="@string/info_is_not_enable" android:key="deleteConfirmationEnable" android:defaultValue="true" />
     <CheckBoxPreference android:title="@string/label_average_data" android:summaryOn="@string/info_is_enable" android:summaryOff="@string/info_is_not_enable" android:key="averageData" android:defaultValue="true" />
-</PreferenceScreen>
+    <CheckBoxPreference android:title="@string/label_goal_line" android:summaryOn="@string/info_is_visible" android:summaryOff="@string/info_is_not_visible" android:key="goalLine" android:defaultValue="true" />
+    <CheckBoxPreference android:title="@string/label_regression_line" android:summaryOn="@string/info_is_visible" android:summaryOff="@string/info_is_not_visible" android:key="regressionLine" android:defaultValue="true" /></PreferenceScreen>