ChartDirector 6.0 (ASP/COM/VB Edition)

Confidence Band

This example demonstrates adding confidence bands to a trend line. It also demonstrates how to display linear regression parameters on the chart.

In linear regression, the data points are assumed to be related by:

y = m * x + c + err

where m and c are constants, and err is a random variable. Linear regression estimates m, c and err based on available data.

As m and c are estimated values, the trend line y = m * x + c may be inaccurate. In ChartDirector, the uncertainties can be represented as a confidence band using TrendLayer.addConfidenceBand. For example, the 95% confidence band means there are 95% probability that the "real" trend line is in that band.

To predict a data point (infer y given x), we use y = m * x + c + err. As m, c and err are estimated values, the predicted data point may be inaccurate. In ChartDirector, the uncertainties can be represented as a prediction band using TrendLayer.addPredictionBand.

Note that the prediction band is always wider than the confidence band because of the extra uncertainties contributed by the err term.

In ChartDirector, the linear regression parameters slope, intercept, correlation coefficient and standard error can be obtained by using TrendLayer.getSlope, TrendLayer.getIntercept, TrendLayer.getCorrelation and TrendLayer.getStdError. (Please refer to appropriate statistics text books for explanation of these values.)

In this example, the linear regression parameters are formatted into a table using CDML, which is then added to the chart as custom text box using BaseChart.addText.

Source Code Listing

[Web Version (in ASP)] aspdemo\confidenceband.asp
 ```<%@ language="vbscript" %> <% Set cd = CreateObject("ChartDirector.API") ' The XY data of the first data series dataX = Array(50, 55, 37, 24, 42, 49, 63, 72, 83, 59) dataY = Array(3.6, 2.8, 2.5, 2.3, 3.8, 3.0, 3.8, 5.0, 6.0, 3.3) ' Create a XYChart object of size 450 x 420 pixels Set c = cd.XYChart(450, 420) ' Set the plotarea at (55, 65) and of size 350 x 300 pixels, with white background and a light grey ' border (0xc0c0c0). Turn on both horizontal and vertical grid lines with light grey color ' (0xc0c0c0) Call c.setPlotArea(55, 65, 350, 300, &Hffffff, -1, &Hc0c0c0, &Hc0c0c0, -1) ' Add a title to the chart using 18 point Times Bold Itatic font. Call c.addTitle("Server Performance", "timesbi.ttf", 18) ' Add titles to the axes using 12pt Arial Bold Italic font Call c.yAxis().setTitle("Response Time (sec)", "arialbi.ttf", 12) Call c.xAxis().setTitle("Server Load (TPS)", "arialbi.ttf", 12) ' Set the axes line width to 3 pixels Call c.yAxis().setWidth(3) Call c.xAxis().setWidth(3) ' Add a scatter layer using (dataX, dataY) Call c.addScatterLayer(dataX, dataY, "", cd.DiamondSymbol, 11, &H008000) ' Add a trend line layer for (dataX, dataY) Set trendLayer = c.addTrendLayer2(dataX, dataY, &H008000) ' Set the line width to 3 pixels Call trendLayer.setLineWidth(3) ' Add a 95% confidence band for the line Call trendLayer.addConfidenceBand(0.95, &H806666ff) ' Add a 95% confidence band (prediction band) for the points Call trendLayer.addPredictionBand(0.95, &H8066ff66) ' Add a legend box at (50, 30) (top of the chart) with horizontal layout. Use 10pt Arial Bold Italic ' font. Set the background and border color to Transparent. Set legendBox = c.addLegend(50, 30, False, "arialbi.ttf", 10) Call legendBox.setBackground(cd.Transparent) ' Add entries to the legend box Call legendBox.addKey("95% Line Confidence", &H806666ff) Call legendBox.addKey("95% Point Confidence", &H8066ff66) ' Display the trend line parameters as a text table formatted using CDML Set textbox = c.addText(56, 65, _ "<*block*>Slope<*br*>Intercept<*br*>Correlation<*br*>Std Error<*/*> <*block*>" & _ FormatNumber(trendLayer.getSlope(), 4) & " sec/tps<*br*>" & FormatNumber( _ trendLayer.getIntercept(), 4) & " sec<*br*>" & FormatNumber(trendLayer.getCorrelation(), 4) & _ "<*br*>" & FormatNumber(trendLayer.getStdError(), 4) & " sec<*/*>", "arialbd.ttf", 8) ' Set the background of the text box to light grey, with a black border, and 1 pixel 3D border Call textbox.setBackground(&Hc0c0c0, 0, 1) ' Output the chart Response.ContentType = "image/png" Response.BinaryWrite c.makeChart2(cd.PNG) Response.End %>```

[Windows Version (in Visual Basic)] vbdemo\confidenceband.cls
 ```Public Sub createChart(viewer As Object, chartIndex As Integer) Dim cd As New ChartDirector.API ' The XY data of the first data series Dim dataX() dataX = Array(50, 55, 37, 24, 42, 49, 63, 72, 83, 59) Dim dataY() dataY = Array(3.6, 2.8, 2.5, 2.3, 3.8, 3.0, 3.8, 5.0, 6.0, 3.3) ' Create a XYChart object of size 450 x 420 pixels Dim c As XYChart Set c = cd.XYChart(450, 420) ' Set the plotarea at (55, 65) and of size 350 x 300 pixels, with white background and a light ' grey border (0xc0c0c0). Turn on both horizontal and vertical grid lines with light grey color ' (0xc0c0c0) Call c.setPlotArea(55, 65, 350, 300, &Hffffff, -1, &Hc0c0c0, &Hc0c0c0, -1) ' Add a title to the chart using 18 point Times Bold Itatic font. Call c.addTitle("Server Performance", "timesbi.ttf", 18) ' Add titles to the axes using 12pt Arial Bold Italic font Call c.yAxis().setTitle("Response Time (sec)", "arialbi.ttf", 12) Call c.xAxis().setTitle("Server Load (TPS)", "arialbi.ttf", 12) ' Set the axes line width to 3 pixels Call c.yAxis().setWidth(3) Call c.xAxis().setWidth(3) ' Add a scatter layer using (dataX, dataY) Dim scatterLayer As ScatterLayer Set scatterLayer = c.addScatterLayer(dataX, dataY, "", cd.DiamondSymbol, 11, &H008000) ' tool tip for scatter layer Call scatterLayer.setHTMLImageMap("", "", "title='Response time at {x} TPS: {value} sec'") ' Add a trend line layer for (dataX, dataY) Dim trendLayer As TrendLayer Set trendLayer = c.addTrendLayer2(dataX, dataY, &H008000) ' Set the line width to 3 pixels Call trendLayer.setLineWidth(3) ' Add a 95% confidence band for the line Call trendLayer.addConfidenceBand(0.95, &H806666ff) ' Add a 95% confidence band (prediction band) for the points Call trendLayer.addPredictionBand(0.95, &H8066ff66) ' tool tip for trend layer Call trendLayer.setHTMLImageMap("", "", _ "title='Slope = {slope|4} sec/TPS; Intercept = {intercept|4} sec'") ' Add a legend box at (50, 30) (top of the chart) with horizontal layout. Use 10pt Arial Bold ' Italic font. Set the background and border color to Transparent. Dim legendBox As LegendBox Set legendBox = c.addLegend(50, 30, False, "arialbi.ttf", 10) Call legendBox.setBackground(cd.Transparent) ' Add entries to the legend box Call legendBox.addKey("95% Line Confidence", &H806666ff) Call legendBox.addKey("95% Point Confidence", &H8066ff66) ' Display the trend line parameters as a text table formatted using CDML Dim textbox As ChartDirector.TextBox Set textbox = c.addText(56, 65, _ "<*block*>Slope<*br*>Intercept<*br*>Correlation<*br*>Std Error<*/*> <*block*>" & _ FormatNumber(trendLayer.getSlope(), 4) & " sec/tps<*br*>" & FormatNumber( _ trendLayer.getIntercept(), 4) & " sec<*br*>" & FormatNumber(trendLayer.getCorrelation(), 4 _ ) & "<*br*>" & FormatNumber(trendLayer.getStdError(), 4) & " sec<*/*>", "arialbd.ttf", 8) ' Set the background of the text box to light grey, with a black border, and 1 pixel 3D border Call textbox.setBackground(&Hc0c0c0, 0, 1) ' Output the chart Set viewer.Picture = c.makePicture() ' include tool tip for the chart viewer.ImageMap = c.getHTMLImageMap("clickable") End Sub```