more bug fixes

core/src/processing/data/FloatDict.java

@@ -411,13 +411,21 @@ public class FloatDict {
 
   public String minKey() {
     checkMinMax("minKey");
-    return keys[minIndex()];
+    int index = minIndex();
+    if (index == -1) {
+      return null;
+    }
+    return keys[index];
   }
 
 
   public float minValue() {
     checkMinMax("minValue");
-    return values[minIndex()];
+    int index = minIndex();
+    if (index == -1) {
+      return Float.NaN;
+    }
+    return values[index];
   }
 
 
@@ -452,17 +460,25 @@ public class FloatDict {
   }
 
 
-  /** The key for a max value. */
+  /** The key for a max value, or null if everything is NaN (no max). */
   public String maxKey() {
     checkMinMax("maxKey");
-    return keys[maxIndex()];
+    int index = maxIndex();
+    if (index == -1) {
+      return null;
+    }
+    return keys[index];
   }
 
 
-  /** The max value. */
+  /** The max value. (Or NaN if they're all NaN.) */
   public float maxValue() {
     checkMinMax("maxValue");
-    return values[maxIndex()];
+    int index = maxIndex();
+    if (index == -1) {
+      return Float.NaN;
+    }
+    return values[index];
   }