While this is a known limitation of the legend, you can programmatically set the visibility of charts larger than index 256 using LabVIEW property nodes.
- Right click on the chart and go to Create»Property Node»Active Plot
- Drop the Property Node on the block diagram
- Expand the Property Node by hovering over it with the mouse and dragging down the blue dot that appears on the bottom
- Click on the new box and select Visible
- Right click on the property node and select Change All to Write
You can now assign the plot you would like to make visible to the active plot property and set the visibility to true. The final property node configuration can be shown in the code snippet below: