FYI ... I was also having some trouble with the 24 hour fixed time. I am editing the source code form a dashboard so I hope this helps I chose to go another route and it worked well for me.
I made a timechart that would grab data for a specific 24 hour period (previous day hardcoded by earliest/latest time), and would bucket results in 5 minute buckets, fill the value null with "zero", do a fixed range (this is the command that makes the range fixed in that 24 hour period)... then the rest is just some fun custom stuff. color etc...
You just need to replace the search, earliest/latest time, and max(XXX) to the custom field you are wanting to chart.
<row>
<panel>
<title>TITLE OF CHART</title>
<chart>
<searchString>
YOURSEARCH FOR SERVER/USER/FOO/BAR etc... earliest=3/12/2015:00:00:00 latest=3/13/2015:00:00:00
| bucket _time span="5m" | fillnull value=NoVal | timechart fixedrange=t span="5m" max(YOURFIELD)
</searchString>
<option name="rowNumbers">undefined</option>
<option name="charting.chart">line</option>
<option name="charting.chart.nullValueMode">gaps</option>
<option name="charting.chart.stackMode">stacked</option>
<option name="charting.drilldown">all</option>
<option name="charting.legend.placement">none</option>
<option name="charting.seriesColors">[0x000000]</option>
<option name="charting.drilldown">all</option>
<option name="charting.axisLabelsX.majorLabelStyle.rotation">0</option>
<option name="charting.axisTitleX.visibility">visible</option>
<option name="charting.axisTitleY.visibility">visible</option>
<option name="charting.axisTitleY2.visibility">visible</option>
<option name="charting.axisX.scale">linear</option>
<option name="charting.axisY.scale">linear</option>
<option name="charting.axisY2.enabled">false</option>
<option name="charting.axisY2.scale">inherit</option>
<option name="charting.chart.bubbleMaximumSize">50</option>
<option name="charting.chart.bubbleMinimumSize">10</option>
<option name="charting.chart.bubbleSizeBy">area</option>
<option name="charting.chart.sliceCollapsingThreshold">0.01</option>
<option name="charting.chart.style">shiny</option>
<option name="charting.layout.splitSeries">0</option>
<option name="charting.legend.labelStyle.overflowMode">ellipsisMiddle</option>
</chart>
</panel>
</row>
For the same code just in a New search you can try this and click on visiualizations...
YOUR SERVER/FOO earliest=3/12/2015:00:00:00 latest=3/13/2015:00:00:00
| YOUR CUSTOM FIELD ...
| bucket _time span="5m"
| fillnull value=NoVal
| timechart fixedrange=t span="5m" max(YOURFIELD)
Good luck
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