title: BarChartView xAxis label doesn't display all labels
body: <p>I have a <code>BarChartView</code> with 31 bars, and thus 31 labels. I add everything and set it up according to examples and it works as it should - except that the BarChartView only displays each 3rd label. So I try to print out 1, 2, 3, 4, 5, 6,..., 30, 31 but It prints 1, 4, 7, 11,..., 31. How can I change this? I don't even need to print all labels, all I really need is the first and last label, so 1 and 31, but I can't for the life of me figure out how to set this.</p>
  
  <p>I've tried setting the label to <code>nil</code> for each bar that is not the first or the last, but that doesn't do anything either. I believe the reason it prints each 3rd label is because it can't fit them all in, correct? My setup for the <code>BarChartView</code> is:</p>
  
  <pre><code>    barChartView.descriptionText = ""
      barChartView.noDataTextDescription = ""
      barChartView.drawGridBackgroundEnabled = false
      barChartView.xAxis.drawAxisLineEnabled = false
      barChartView.xAxis.drawGridLinesEnabled = false
      barChartView.xAxis.drawLabelsEnabled = true
      barChartView.drawBordersEnabled = false
      barChartView.leftAxis.enabled = false
      barChartView.rightAxis.enabled = false
      barChartView.legend.enabled = false
      barChartView.xAxis.labelPosition = .Bottom
      barChartView.xAxis.labelTextColor = UIColor.whiteColor()
      barChartView.xAxis.labelFont = UIFont(name: timesNewRoman, size: barChartView.xAxis.labelFont.pointSize)!
      barChartView.dragEnabled = false
      barChartView.highlightEnabled = false
      barChartView.scaleXEnabled = false
      barChartView.scaleYEnabled = false
  </code></pre>
  
  <p>and I add the <code>xValues</code> like this:</p>
  
  <pre><code>for (index, value) in plotData.enumerate() {
          var xValue: String?
          xValue = index == 0 || index == plotData.count-1 ? "\(index + 1)" : nil
  
  
          barChartData.addXValue(xValue)
          let dataEntry = BarChartDataEntry(value: Double(value), xIndex: index)
  
          dataEntries.append(dataEntry)
  }
  </code></pre>