TrackStatistics and ChartView: handle altitude and speed as optional with null as default value.

Then only the relevant part of the y-axis is shown.

Fixes #795.

src/main/java/de/dennisguse/opentracks/content/TrackDataListener.java

@@ -17,7 +17,9 @@
 package de.dennisguse.opentracks.content;
 
 import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 
+import de.dennisguse.opentracks.content.data.Altitude;
 import de.dennisguse.opentracks.content.data.Marker;
 import de.dennisguse.opentracks.content.data.Speed;
 import de.dennisguse.opentracks.content.data.Track;
@@ -49,7 +51,7 @@ public interface TrackDataListener {
      *
      * @param trackPoint the trackPoint
      */
-    default void onSampledInTrackPoint(@NonNull TrackPoint trackPoint, @NonNull TrackStatistics trackStatistics, Speed smoothedSpeed, double smoothedAltitude_m) {
+    default void onSampledInTrackPoint(@NonNull TrackPoint trackPoint, @NonNull TrackStatistics trackStatistics, Speed smoothedSpeed, @Nullable Altitude smoothedAltitude_m) {
     }
 
     /**

src/main/java/de/dennisguse/opentracks/content/data/Altitude.java

@@ -14,6 +14,8 @@ public abstract class Altitude {
         return altitude_m;
     }
 
+    public abstract Altitude replace(double altitude_m);
+
     public abstract int getLabelId();
 
     public static class WGS84 extends Altitude {
@@ -30,6 +32,11 @@ public abstract class Altitude {
         public static Altitude of(double altitude_m) {
             return new WGS84(altitude_m);
         }
+
+        @Override
+        public Altitude replace(double altitude_m) {
+            return new WGS84(altitude_m);
+        }
     }
 
     public static class EGM2008 extends Altitude {
@@ -46,6 +53,11 @@ public abstract class Altitude {
         public static Altitude of(double altitude_m) {
             return new EGM2008(altitude_m);
         }
+
+        @Override
+        public Altitude replace(double altitude_m) {
+            return new EGM2008(altitude_m);
+        }
     }
 }