c# - Make Chart Legend represent two colors

Question

I created a column chart in my application which look like this:

As you can see the positive values are green and the negative values are red. I need to represent this in the legend. I just don't know how.

What I already tried:

I added CustomItems to the Legend. Here is the code:

Legend currentLegend = chart.Legends.FindByName(chart.Series[series].Legend);
if (currentLegend != null)
{
    currentLegend.LegendStyle   = LegendStyle.Table;
    LegendItem li               = new LegendItem();
    li.Name                     = series;
    li.Color                    = Color.Red;
    li.BorderColor              = Color.Transparent;
    currentLegend.CustomItems.Add(li);
}

This results in the following representation:

I could live with that. But as soon as I add further series to the chart the order of the elements gets destroyed. Here is an example:

I would like to have one of the two options:

1. keep the positive and negative color together
2. or an even better solution could be to have just one tile in the legend which is double colored. Something like this:

Could you please help me solving this issue?

Many thanks in advance!

Answer 1

Yes, you can do that. Note however that you can't really modify the original Legend. So for a perfect result you would need to create a new custom Legend instead.

See here for an example that does that; note especially the positioning..!

But maybe you can get away a little easier; see below!

The first rule to understand is that added LegendItems always go to the end of the list. So you can't keep them together, unless your added Series are at the start. You can do that by using Series.Insert(..) but using those two-color rectangles is much nicer, imo..

To show the graphics you want, simply create them as bitmaps, either on disk or on the fly and store them in the Images collection of the chart:

Legend L = chart1.Legends[0];
Series S = chart1.Series[0];

// either load an image from disk (or resources)
Image img = Image.FromFile(someImage);

// or create it on the fly:
Bitmap bmp = new Bitmap(32, 14);
using (Graphics G = Graphics.FromImage(bmp))
{
    G.Clear(Color.Red);
    G.FillPolygon(Brushes.LimeGreen, new Point[] { new Point(0,0), 
        new Point(32,0), new Point(0,14)});
}

Now add it to the chart's NamedImage collection:

chart1.Images.Add(new NamedImage("dia", bmp));

Now you can create as many LegendItems as you need:

LegendItem newItem = new LegendItem();
newItem.ImageStyle = LegendImageStyle.Rectangle;
newItem.Cells.Add(LegendCellType.Image, "dia", ContentAlignment.MiddleLeft);
newItem.Cells.Add(LegendCellType.Text, S.Name, ContentAlignment.MiddleLeft);

And add them to the Legend:

L.CustomItems.Add(newItem);

Unfortunately you can't delete the original item.

What you can do, besides creating a new Legend from scratch, is this:

Clear the text like this:

S.LegendText = " "; // blank, not empty!

As you have set the Colors of all the DataPoints anyway, you can also get rid of the blue rectangle:

S.Color = Color.Transparent;

This will also make all points without colors transparent, so make sure to color them all!

Note that some space in the Legend it still taken!

Here is the result, with a few colored points and your line series added: