cargo-trend is a cargo subcommand to generate trend graph of dependent crates.
For example, the following comand willl generate a trend graph of some error management crates.
To me it's interesting to see that it's very rare for crates to have a downward trend. The biggest one is cargo trend gcc cc, and even that is a modest one. cargo trend rustc-serialize serde barely moved. time chrono and tempdir tempfile show the deprecated ones are still growing.
One thing I found useful was looking at dependencies as a fraction of size of crates.io. This gives a much better indication of when a crate is "at its peak" rather than still expanding as the absolute numbers might suggest.
Two other features that would be neat to see in cargo trend: optionally count transitive dependencies (not just direct dependencies), and be able to break things down by crate version so you can graph e.g. serde:0.8, serde:0.9, serde:1.0.