title: tick interval not set properly
body: <p>.When i set the , tick amount, min and max,  the interval is not calculated properly in some cases for highcharts.Below is an example where its calculated properly. the interval is set at 0,1,2,3
  <a href="https://jsfiddle.net/noel3090/ng0sj97q/1/" rel="nofollow noreferrer">fiddle</a></p>
  <pre><code>       xAxis: [{
           min: 0,
           gridLineWidth: 1,
           endOnTick: true,
           tickAmount: 4,
           max:2.5
         }],
  </code></pre>
  <p>Where as in this fiddle  <a href="https://jsfiddle.net/noel3090/v6xzua13/6/" rel="nofollow noreferrer">fiddle 2</a></p>
  <pre><code>       xAxis: [{
            min: 0,
            gridLineWidth: 1,
            endOnTick: true,
            tickAmount: 3,
            max:14
         }],
  </code></pre>
  <p>the interval is set to 8.
  Also its not taking the max value.
  I want the interval set at 7 and not 8 and also max value to be 14 and not 16.</p>