jqplot tickInterval issue

HTML

<script src="https://bitbucket.org/cleonello/jqplot/raw/32bbb38b42ffea9d2e2f985326bc23a7677c65d6/src/jquery.jqplot.js"></script>
<script src="https://bitbucket.org/cleonello/jqplot/raw/32bbb38b42ffea9d2e2f985326bc23a7677c65d6/src/plugins/jqplot.canvasAxisLabelRenderer.js"></script>
<script src="https://bitbucket.org/cleonello/jqplot/raw/32bbb38b42ffea9d2e2f985326bc23a7677c65d6/src/plugins/jqplot.dateAxisRenderer.js"></script>
<script src="https://bitbucket.org/cleonello/jqplot/raw/32bbb38b42ffea9d2e2f985326bc23a7677c65d6/src/plugins/jqplot.canvasAxisTickRenderer.js"></script>
<script src="https://bitbucket.org/cleonello/jqplot/raw/32bbb38b42ffea9d2e2f985326bc23a7677c65d6/src/plugins/jqplot.canvasTextRenderer.js"></script>
<div id="date_range_slider_1"></div>
<div>
  <div style="float:left">Display interval:</div>
  <div style="float:left">
    <select id="display_interval_1">
            <option value="1 month">1 month</option>
            <option value="3 month" selected>3 month</option>
            <option value="6 month">6 month</option>
            <option value="1 year">1 year</option>
        </select>
  </div>
</div>
<div>
  <button type="button" id="calc1">Generate</button>
</div>
<div id="chart1" style="margin-top:20px; margin-left:20px; width:650px; height:400px;"></div>

CSS

/*rules for the plot target div.  These will be cascaded down to all plot elements according to css rules*/

.jqplot-target {
  position: relative;
  color: #666666;
  font-family: "Trebuchet MS", Arial, Helvetica, sans-serif;
  font-size: 1em;
  /*    height: 300px;
    width: 400px;*/
}

/*rules applied to all axes*/

.jqplot-axis {
  font-size: 0.75em;
}

.jqplot-xaxis {
  margin-top: 10px;
}

.jqplot-x2axis {
  margin-bottom: 10px;
}

.jqplot-yaxis {
  margin-right: 10px;
}

.jqplot-y2axis,
.jqplot-y3axis,
.jqplot-y4axis,
.jqplot-y5axis,
.jqplot-y6axis,
.jqplot-y7axis,
.jqplot-y8axis,
.jqplot-y9axis,
.jqplot-yMidAxis {
  margin-left: 10px;
  margin-right: 10px;
}

/*rules applied to all axis tick divs*/

.jqplot-axis-tick,
.jqplot-xaxis-tick,
.jqplot-yaxis-tick,
.jqplot-x2axis-tick,
.jqplot-y2axis-tick,
.jqplot-y3axis-tick,
.jqplot-y4axis-tick,
.jqplot-y5axis-tick,
.jqplot-y6axis-tick,
.jqplot-y7axis-tick,
.jqplot-y8axis-tick,
.jqplot-y9axis-tick,
.jqplot-yMidAxis-tick {
  position: absolute;
  white-space: pre;
}

.jqplot-xaxis-tick {
  top: 0px;
  /* initial position untill tick is drawn in proper place */
  left: 15px;
  /*    padding-top: 10px;*/
  vertical-align: top;
}

.jqplot-x2axis-tick {
  bottom: 0px;
  /* initial position untill tick is drawn in proper place */
  left: 15px;
  /*    padding-bottom: 10px;*/
  vertical-align: bottom;
}

.jqplot-yaxis-tick {
  right: 0px;
  /* initial position untill tick is drawn in proper place */
  top: 15px;
  /*    padding-right: 10px;*/
  text-align: right;
}

.jqplot-yaxis-tick.jqplot-breakTick {
  right: -20px;
  margin-right: 0px;
  padding: 1px 5px 1px 5px;
  /*	background-color: white;*/
  z-index: 2;
  font-size: 1.5em;
}

.jqplot-y2axis-tick,
.jqplot-y3axis-tick,
.jqplot-y4axis-tick,
.jqplot-y5axis-tick,
.jqplot-y6axis-tick,
.jqplot-y7axis-tick,
.jqplot-y8axis-tick,
.jqplot-y9axis-tick {
  left: 0px;
  /* initial position untill tick is drawn in proper place */
  top: 15px;
  /*    padding-left: 10px;*/
...

JavaScript

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