Trend Line Chart

This example demonstrates linear regression trend line fitting. It also demonstrates rotating the x-axis labels by 90 degrees to save axis space.

A trend line is a straight line that best fit certain data points. A trend line is computed using linear regression (the least square method).

In ChartDirector, trend lines are created by using XYChart.addTrendLayer or XYChart.addTrendLayer2. A trend layer only shows the trend line, not the data points. Another layer, such as a line or scatter layer, can be used to show the data points.

In this example, two layers are used. A line layer displays the data points as symbols and draws a line that joins them. A trend layer draws a trend line that best fit the same data points.

Note that the x-axis labels 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.

[JSP Version] jspdemo/trendline.jsp

<%@page import="ChartDirector.*, java.util.*" %>
<%
// The data for the line chart
double[] data = {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
String[] labels = {"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.
XYChart c = new XYChart(500, 320, 0xffccff, 0x000000, 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 8pt Arial font. Set
// the background and border color to Transparent.
c.addLegend(55, 22, false, "", 8).setBackground(Chart.Transparent);

// Add a title box to the chart using 13pt Times Bold Italic font. The text is white (0xffffff) on a
// purple (0x800080) background, with a 1 pixel 3D border.
c.addTitle("Long Term Server Load", "Times New Roman Bold Italic", 13, 0xffffff).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.
c.xAxis().setLabels(labels).setFontAngle(90);

// Add a line layer to the chart
LineLayer lineLayer = c.addLineLayer();

// Add the data to the line layer using light brown color (0xcc9966) with a 7 pixel square symbol
lineLayer.addDataSet(data, 0xcc9966, "Server Utilization").setDataSymbol(Chart.SquareSymbol, 7);

// Set the line width to 2 pixels
lineLayer.setLineWidth(2);

// tool tip for the line layer
lineLayer.setHTMLImageMap("", "", "title='{xLabel}: {value} MBytes'");

// Add a trend line layer using the same data with a dark green (0x008000) color. Set the line width
// to 2 pixels
TrendLayer trendLayer = c.addTrendLayer(data, 0x008000, "Trend Line");
trendLayer.setLineWidth(2);

// tool tip for the trend layer
trendLayer.setHTMLImageMap("", "", "title='Change rate: {slope|2} MBytes/per month'");

// Output the chart
WebChartViewer viewer = new WebChartViewer(request, "chart1");
viewer.setChart(c, Chart.SVG);

// include tool tip for the chart
viewer.setImageMap(c.getHTMLImageMap(""));
%>
<!DOCTYPE html>
<html>
<head>
    <title>Trend Line Chart</title>
    <!-- Include ChartDirector Javascript Library to support chart interactions -->
    <script type="text/javascript" src="cdjcv.js"></script>
</head>
<body style="margin:5px 0px 0px 5px">
    <div style="font:bold 18pt verdana;">
        Trend Line Chart
    </div>
    <hr style="border:solid 1px #000080; background:#000080" />
    <div style="font:10pt verdana; margin-bottom:1.5em">
        <a href="viewsource.jsp?file=<%=request.getServletPath()%>">View Source Code</a>
    </div>
    <!-- ****** Here is the chart image ****** -->
    <%= viewer.renderHTML(response) %>
</body>
</html>