A trend line is a straight line that fits a number of data points computed using linear regression (the least square method).

In ChartDirector, a trend line is created by adding a trend line layer using XYChart.addTrendLayer or XYChart.addTrendLayer2 method.

Typically, a trend line layer is used with other layers for the data points. For example, in this example, two layers are used. A line chart layer draws the line that joins the data points. The trend line layer draws a straight line that best fit the same data points.

Note that the x-axis labels on the above chart are rotated by 90 degrees to save space. This is achieved by using TextBox.setFontAngle of the TextBox object that represents the x-axis label prototype.

[File: phpdemo/trendline.php".]

<?php require_once("../lib/phpchartdir.php"); #The data for the line chart $data = array(50, 55, 47, 34, 42, 49, 63, 62, 73, 59, 56, 50, 64, 60, 67, 67, 58, 59, 73, 77, 84, 82, 80, 91); #The labels for the line chart $labels = array("Jan 2000", "Feb 2000", "Mar 2000", "Apr 2000", "May 2000", "Jun 2000", "Jul 2000", "Aug 2000", "Sep 2000", "Oct 2000", "Nov 2000", "Dec 2000", "Jan 2001", "Feb 2001", "Mar 2001", "Apr 2001", "May 2001", "Jun 2001", "Jul 2001", "Aug 2001", "Sep 2001", "Oct 2001", "Nov 2001", "Dec 2001"); #Create a XYChart object of size 500 x 320 pixels, with a pale purpule #(0xffccff) background, a black border, and 1 pixel 3D border effect. $c = new XYChart(500, 320, 0xffccff, 0x0, 1); #Set the plotarea at (55, 45) and of size 420 x 210 pixels, with white #background. Turn on both horizontal and vertical grid lines with light grey #color (0xc0c0c0) $c->setPlotArea(55, 45, 420, 210, 0xffffff, -1, -1, 0xc0c0c0, -1); #Add a legend box at (55, 25) (top of the chart) with horizontal layout. Use 8 #pts Arial font. Set the background and border color to Transparent. $legendObj = $c->addLegend(55, 25, false, "", 8); $legendObj->setBackground(Transparent); #Add a title box to the chart using 13 pts Times Bold Italic font. The text is #white (0xffffff) on a purple (0x800080) background, with a 1 pixel 3D border. $titleObj = $c->addTitle("Long Term Server Load", "timesbi.ttf", 13, 0xffffff); $titleObj->setBackground(0x800080, -1, 1); #Add a title to the y axis $c->yAxis->setTitle("MBytes"); #Set the labels on the x axis. Rotate the font by 90 degrees. $labelsObj = $c->xAxis->setLabels($labels); $labelsObj->setFontAngle(90); #Add a line layer to the chart $lineLayer = $c->addLineLayer(); #Add the data to the line layer using light brown color (0xcc9966) with a 7 #pixel square symbol $dataSetObj = $lineLayer->addDataSet($data, 0xcc9966, "Server Utilization"); $dataSetObj->setDataSymbol(SquareSymbol, 7); #Set the line width to 2 pixels $lineLayer->setLineWidth(2); #Add a trend line layer using the same data with a dark green (0x008000) color. #Set the line width to 2 pixels $trendLayerObj = $c->addTrendLayer($data, 0x8000, "Trend Line"); $trendLayerObj->setLineWidth(2); #output the chart header("Content-type: image/png"); print($c->makeChart2(PNG)); ?>