private RenderTargetBitmap createBitmap(BaseSymbol symbol)
        {
            if (symbolCache.ContainsKey(symbol))
                return symbolCache[symbol];

            FrameworkElement element;
            switch (symbol.Type)
            {
                default: // ToDo: support different types
                    element = new Pyramid {Color = symbol.Color};
                    break;
            }
            element.Width = element.Height = symbol.Size;

            element.Measure(new Size(element.Width, element.Height));
            element.Arrange(new Rect(0, 0, element.Width, element.Height));

            var bitmap = new RenderTargetBitmap((int)element.Width, (int)element.Height, 96, 96, PixelFormats.Pbgra32);
            bitmap.Render(element);
            bitmap.Freeze();

            symbolCache.Add(symbol, bitmap);
            return bitmap;
        }