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  Plotting live data in real time over 24 hours.
Posted by Harlow Pinson on Aug-19-2011 01:03
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Hi Peter.

I've run into a problem and wonder if you could offer some advice.

My application is a line chart, with a new value plotted each hour over 24 hours.  A data point is added hourly to the db, just before I chart it.

I find that if all elements of the array exist (i.e. 24 hours) the chart will render.

But if values are missing (ie. the 9:00 value has not yet been written to the db) the chart will not render.

Suggestions?  Here is an outline of my PHP code approach:

<?php

// SQL Query.
$result = mssql_query("SELECT timestamp, field1 FROM table WHERE timestamp LIKE '$today%'");

// Assign field values from query results to an array.
while($row = mssql_fetch_array($result))
  {
  $myarray[] = $row['field1'];
  }


# The data for the line chart
$data0 = array($myarray[0],$myarray[1],$myarray[2],$myarray[3],$myarray[4],$myarray[5],$myarray[6],$myarray[7],$myarray[8],$myarray[9],$myarray[10],$myarray[11],$myarray[12],$myarray[13],$myarray[14],$myarray[15],$myarray[16],$myarray[17],$myarray[18],$myarray[19],$myarray[20],$myarray[21],$myarray[22],$myarray[23]);

// ... formatting removed for this example.

# Add a line layer to the chart
$layer = $c->addLineLayer2();

# Set the line width to 3 pixels
$layer->setLineWidth(3);

# Add the three data sets to the line layer, using circles, diamands and X shapes as
# symbols
$dataSetObj = $layer->addDataSet($data0, 0xff0000, "Flow Diverted (CFS)");
$dataSetObj->setDataSymbol(CircleSymbol, 9);

# Output the chart
header("Content-type: image/png");
print($c->makeChart2(PNG));


?>

  Re: Plotting live data in real time over 24 hours.
Posted by Peter Kwan on Aug-19-2011 23:50
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Hi Harlow,

There are many reasons why the chart cannot be rendered. For example, your PHP may be configured to output warning or error messages when you use undefined values. (The $myarray[15] is undefined if you only have 9 values, but your code use it anyway to construct the $data0 array.)

If I were you, I would use:

$data0 = $myarray;

.....

$c->xAxis->setLinearScale(0, 24, 4);

(Note that the above code assumes $myarray exists. In your current code, there is no guarantee that $myarray exists. You may want to add one more line "$myarray=array();" at the start of your code to make sure $myarray exists.

Hope this can help.

Regards
Peter Kwan