Hello @RayLin
That's quiet interesting question!
Unfortunately, there is no such function in the Qiime2 by default.
As an alternative, you can use heatmap directly with your feature table.
But it bothered me for a while, and I wanted to try one trick to create an asvbarplot, although never implemented it until you asked the same question!
So, here is an illegal solution 
:
1 Export your taxonomy as a tsv file:
qiime tools export \
--input-path taxonomy.qza \
--output-path ./
2 Open resulted taxonomy.tsv file in any editor you like and copy Feature ID column values (ASV hashes). Replace taxonomy annotations in Taxon column by ASV IDs. Save changes into the same file. Or, you can rename taxonomy annotations in taxon column to ASV1, ASV2.. ASVn if you would like so on this step
3 Import this file back into the Qiime:
qiime tools import \
--type 'FeatureData[Taxonomy]' \
--input-path taxonomy.tsv \
--output-path mock_taxa_asv.qza
4 Create an ASVbarplot:
qiime taxa barplot \
--i-table feature-table.qza \
--i-taxonomy mock_taxa_asv.qza \
--m-metadata-file metadata.tsv \
--o-visualization asv-barplot.qzv
Which will create barplot with ASVs instead of taxa
