Hey there @Alan_Chan - looks like you found a bug! 
Thanks for sharing your viz - the samples are all there, they just aren't visible in the rendered plot, due to a misconfigured html element (open issue here):
I don't have a good workaround for you - I'm sorry! The best I have is this workaround, in Firefox:
Right click on the legend and select "Inspect element". A new window will load in your browser with info about the document. Scroll to this line:
Remove the text overflow-y: hidden from the style attr:
Now you can close this new window - you should be able to scroll the legend up and down.
Sorry, stay tuned for a fix! :qiime2: