fix labels when all same value

@@ -66,16 +66,28 @@
 {
     CGFloat yStep = (_yValueMax - _yValueMin) / _yLabelNum;
     CGFloat yStepHeight = _chartCavanHeight / _yLabelNum;
+    NSString *yLabelFormat = self.yLabelFormat ?: @"%1.f";
 
-    if (_showLabel) {
+    if (yStep == 0.0) {
+        PNChartLabel *minLabel = [[PNChartLabel alloc] initWithFrame:CGRectMake(0.0, _chartCavanHeight, _chartMargin, _yLabelHeight)];
+        minLabel.text = [NSString stringWithFormat:yLabelFormat, 0.0];
+        [self addSubview:minLabel];
+
+        PNChartLabel *midLabel = [[PNChartLabel alloc] initWithFrame:CGRectMake(0.0, _chartCavanHeight/2, _chartMargin, _yLabelHeight)];
+        midLabel.text = [NSString stringWithFormat:yLabelFormat, _yValueMax];
+        [self addSubview:midLabel];
 
+        PNChartLabel *maxLabel = [[PNChartLabel alloc] initWithFrame:CGRectMake(0.0, 0.0, _chartMargin, _yLabelHeight)];
+        maxLabel.text = [NSString stringWithFormat:yLabelFormat, _yValueMax * 2];
+        [self addSubview:maxLabel];
+
+    } else {
         NSInteger index = 0;
         NSInteger num = _yLabelNum + 1;
 
         while (num > 0) {
             PNChartLabel *label = [[PNChartLabel alloc] initWithFrame:CGRectMake(0.0, (_chartCavanHeight - index * yStepHeight), _chartMargin, _yLabelHeight)];
             [label setTextAlignment:NSTextAlignmentRight];
-            NSString *yLabelFormat = self.yLabelFormat ? self.yLabelFormat : @"%1.f";
             label.text = [NSString stringWithFormat:yLabelFormat, _yValueMin + (yStep * index)];
             [self addSubview:label];
             index += 1;
@@ -227,7 +239,11 @@
 
             yValue = chartData.getData(i).y;
 
+            if (!(_yValueMax - _yValueMin)) {
+                innerGrade = 0.5;
+            } else {
                 innerGrade = (yValue - _yValueMin) / (_yValueMax - _yValueMin);
+            }
 
             int x = 2 * _chartMargin +  (i * _xLabelWidth);
             int y = _chartCavanHeight - (innerGrade * _chartCavanHeight) + (_yLabelHeight / 2);
@@ -310,7 +326,6 @@
         // setup the color of the chart line
         if (chartData.color) {
             chartLine.strokeColor = [chartData.color CGColor];
-            pointLayer.strokeColor = [chartData.color CGColor];
         }
         else {
             chartLine.strokeColor = [PNGreen CGColor];