The chart isn't wrong. It's just measuring something different than the metric str4d is referring to.

Underneath that pie chart, there is a caption: "Transparent value (stored in t-addresses) vs shielded value (stored in z-addresses), in ZEC." In other words, that pie chart shows the number of ZEC that are currently residing in transparent vs shielded addresses at the specific point in time that you load that page.

The Advanced Network Stats page - https://explorer.zcha.in/statistics/network - has a box labelled "Shielded Transaction Percentage", which indicates what percentage of transactions involve a shielded value. You can see more details for different amounts of time on https://explorer.zcha.in/statistics/usage

Perhaps it could have something to do with termonology (%value vs %transactions).

For value I get: 42588 / (891015+222753) = 3.8% Shielded Value / (Cumulative Miner's Reward + Cumulative Founder's Reward)

Perhaps it is 28% of transactions are shielded worth only 4% of zec value?

Note that the number of prior shielded transactions (not the proportion, and not the value) is what is actually relevant to the privacy of new shielded transactions. Roughly speaking, the privacy you get with Zcash is comparable to what you would get with Monero if you could use all previous shielded transactions (over 120000 of them, currently) as mixins. That's why the criticisms of Zcash based on the percentage use of shielding (either by transactions or value) are totally missing the point.

-- Daira Hopwood (Zcash developer)

The number of note commitments can be found using 'zcash-cli getblockchaininfo' and is currently 301068 commitments, i.e. 150534 JoinSplits (so a bit more than the 120000 I said).