I have do more tests with svg exporter (the same problem is for pdf exporter) trying to export more complicated diagrams and i have observed an memory ussage issue.
With each call diagram_DoubleClicked the memory usage increases and after the finished export operation memory is not being freed.

    public partial class MainWindow : Window
    {
        public List<Diagram> diagrams { get; set; }

        public MainWindow()
        {
            InitializeComponent();

            var rand = new Random();
            diagrams = new List<Diagram>();

            for (int j = 0; j < 40; j++)
            {
                var diag = new Diagram();

                for (int i = 0; i < 100; i++)
                {
                    var shapeNode = new ShapeNode()
                    {
                        Shape = new Shape(new ElementTemplate[]
                        {
                            new LineTemplate(0, 0, 100, 0),
                            new LineTemplate(100, 0, 100, 100),
                            new LineTemplate(100, 100, 0, 100),
                        }, FillRule.EvenOdd),
                    };

                    shapeNode.AddLabel(new NodeLabel() { Text = "Test" });

                    diag.Nodes.Add(shapeNode);
                    diag.Nodes.Last().Move(rand.Next(0, 1000), rand.Next(0, 1000));
                }

                diagrams.Add(diag);
            }
        }

        private void diagram_DoubleClicked(object sender, DiagramEventArgs e)
        {
            var exporter = new PdfExporter();

            foreach (var diag in diagrams)
                exporter.Export(diag, @"D:\temp\diagram.svg");
        }
    }
 

It's better.... but not perfect.
Why memory allocated to ShapeNodes cache is not being freed after saving the output file ?

If you really need to free that memory after full rendering of offscreen diagrams, you could try exporting their copies instead, or after export clear each diagram and reload from a file or your data. Otherwise you'll have the window -> diagrams array -> nodes -> cached geometries references from root preventing GC from reclaiming the added cache.

Maybe it should, but we've been developing this as UI control that has one or two on-screen instances, that users might occasionally export to PDF. Most of the WeakReferences that grow the heap are WPF internals anyway from what we can tell, and .NET will recycle them when memory needed.

If you are looking to create a PDF generation service for large number of off-screen diagrams with lower memory footprint, you could try the diagramming.dll from our ASP.NET and WinForms controls (it's shared between them); this same test never takes more than 50MB with it -

public partial class Form1 : Form
{
	public Form1()
	{
		InitializeComponent();

		var rand = new Random();
		diagrams = new List<Diagram>();

		for (int j = 0; j < 40; j++)
		{
			var diag = new Diagram();

			for (int i = 0; i < 100; i++)
			{
				var shapeNode = new ShapeNode()
				{
					Shape = new Shape(new ElementTemplate[]
					{
						new LineTemplate(0, 0, 100, 0),
						new LineTemplate(100, 0, 100, 100),
						new LineTemplate(100, 100, 0, 100),
					}, System.Drawing.Drawing2D.FillMode.Alternate),
				};

				shapeNode.AddLabel(new NodeLabel() { Text = "Test" });

				diag.Nodes.Add(shapeNode);
				diag.Nodes.Last().Move(rand.Next(0, 1000), rand.Next(0, 1000));
			}

			diag.ResizeToFitItems(5);
			diagrams.Add(diag);
		}
	}

	public List<Diagram> diagrams { get; set; }

	private void button1_Click(object sender, EventArgs e)
	{
		var exporter = new PdfExporter();

		foreach (var diag in diagrams)
			exporter.Export(diag, "diagram.pdf");
	}
} 

https://www.nuget.org/packages/MindFusion.Diagramming/

Regards,
Slavcho
Mindfusion