Display Percentages on a Pie Chart

Building on a previous post about how to create a pie chart in ASP.NET, let’s look at how to display percentages on a pie chart.  This is surprisingly difficult because it is unclear which chart properties need to be set in order to achieve the look that we’re interested in.

The most common way to display percentages on a pie chart is using the pie labels themselves:

Here is what it looks like after we display the labels outside the pie chart:

...
using System.Web.UI.DataVisualization.Charting;

public partial class PieChartPercentageLabels : System.Web.UI.Page
{   
     protected void Page_Load(object sender, EventArgs e) 
     {        
        // Insert code to create basic pie chart
        // See my blog post entitled "Pie Charts in ASP.NET" for full source code

        // Set the pie labels to be drawn outside of the pie chart
        this.Chart2.Series[0]["PieLabelStyle"] = "Outside";

        // Set these other two properties so that you can see the connecting lines
        this.Chart2.Series[0].BorderWidth = 1;
        this.Chart2.Series[0].BorderColor = System.Drawing.Color.FromArgb(26, 59, 105);

        // Set the pie label as well as legend text to be displayed as percentage
        // The P2 indicates a precision of 2 decimals
        this.Chart2.Series[0].Label = "#PERCENT{P2}";
     }
}

Display Percentages in the Legend of a Pie Chart

Showing percentages on the pie chart is useful to help give context to the slices of the pie, but we still need to see the categories (in this case, the sales names) on the pie somewhere.  One potential solution is to keep the category names as outside labels and show the percentages in the legend.

...
using System.Web.UI.DataVisualization.Charting;

public partial class PieChartPercentageLabels : System.Web.UI.Page
{   
     protected void Page_Load(object sender, EventArgs e) 
     {        
        // Insert code to create basic pie chart
        // See my blog post entitled "Pie Charts in ASP.NET" for full source code

         // Set pie labels to be outside the pie chart
         this.Chart2.Series[0]["PieLabelStyle"] = "Outside";

         // Set border width so that labels are shown on the outside
         this.Chart2.Series[0].BorderWidth = 1;
         this.Chart2.Series[0].BorderColor = System.Drawing.Color.FromArgb(26, 59, 105);

         // Add a legend to the chart and dock it to the bottom-center
         this.Chart2.Legends.Add("Legend1");
         this.Chart2.Legends[0].Enabled = true;
         this.Chart2.Legends[0].Docking = Docking.Bottom;
         this.Chart2.Legends[0].Alignment = System.Drawing.StringAlignment.Center;

         // Set the legend to display pie chart values as percentages
         // Again, the P2 indicates a precision of 2 decimals
         this.Chart2.Series[0].LegendText = "#PERCENT{P2}";

         // By sorting the data points, they show up in proper ascending order in the legend
         this.Chart2.DataManipulator.Sort(PointSortOrder.Descending, Chart2.Series[0]);
     }
}

Some points of interest for this chart:

• To maximize space on a pie chart, I always place the legend below the pie instead of on the right side.
• I tried to give the legend entries more meaning by sorting the data points in descending order on the chart.
• If your pie chart is too complicated to understand after you have performed these steps, you may want to consider displaying your data using a different chart type.

Lastly, if you disable pie labels and use a creative mix of keywords that Alex documented on his blog, you can add category names and percentages in the legend:

...
using System.Web.UI.DataVisualization.Charting;

public partial class PieChartPercentageLabels : System.Web.UI.Page
{   
     protected void Page_Load(object sender, EventArgs e) 
     {        
         // Insert code to create basic pie chart
         // See the post entitled "Pie Charts in ASP.NET" for full source code

         // Set pie labels to be outside the pie chart
         this.Chart2.Series[0]["PieLabelStyle"] = "Disabled";

         // Add a legend to the chart and dock it to the bottom-center
         this.Chart2.Legends.Add("Legend1");
         this.Chart2.Legends[0].Enabled = true;
         this.Chart2.Legends[0].Docking = Docking.Bottom;
         this.Chart2.Legends[0].Alignment = System.Drawing.StringAlignment.Center;

         // Show labels in the legend in the format "Name (### %)"
         this.Chart2.Series[0].LegendText = "#VALX (#PERCENT)";

         // By sorting the data points, they show up in proper ascending order in the legend
         this.Chart2.DataManipulator.Sort(PointSortOrder.Descending, Chart2.Series[0]);
     }
}

For anyone using SSRS, check out the MSDN article on How to: Display Percentage Values on a Pie Chart.

Feel free to post questions if this is unclear…