Enable transparent as a color option with Pattern fill.
“Solid fill” offers transparency; “Gradient fill” offers transparency; “Picture or texture fill” offers transparency; yet “Pattern fill” does not. It is the sole fill means lacking this “color” offering (See attached.).
Transparency as a color option would allow some interesting visualizations, especially when graphing on a secondary axis. Transparent for Background Color has the most obvious use, yet implementing as the foreground color should be provided as well to empower the user.
Implementing this should be very straightforward.
In Visio enable developer options. Click your shape. Click show shapesheet in the developer tab. Scroll down to Fill and set "FillBkgrndTrans" to 100%. Copy into Excel as Picture. Happy days.
I found another workaround. If, in the chart, you use a "Picture or texture fill" and choose a File that is a .png with a transparent background, those areas will be transparent in the chart.
Susan A commented
There is another workaround. Create a shape of the dimensions you want. put in the pattern fill you want. save the shape as a picture. Then use that picture as a picture fill, and adjust the transparency.
It's annoying but there's a workaround by using the Clipboard option within the 'Picture or Texture Fill'.
Set up your shape to the correct size, shape and colour with the desired pattern fill (but with no outline). Then copy / ctrl + c it.
Now change the fill to 'Picture or Texture Fill' and click Clipboard. This inserts the copied pattern fill and allows you to change the transparency of it.