Here is some code to record difficulty. I posted it a month ago, but this is a better version. I copy and past this output to a spreadsheet to get the difficulty chart. The recorded time to get a block is as it arrives to your node which will have some error. It can have up to an additional 10 seconds of error. The official data can be found at https://explorer.testnet.z.cash Maybe there’s some way to get the data in a chartable format from there.
When difficulty says “1” it appears to be the previous difficulty, not actually 1. For example, it takes the same amount of time to solve d=1400 as d=1 when it is switching back and forth between the two. I never could get the devs to comment on that except to say going back to 1 will not exist in version 1.0.
#!/bin/bash
# WARNING: this may slow your miner
# Logs time, "apparent" secs to get block, block num, and difficulty
# Execute this file log.sh with "bash log.sh".
# Assumes zcash folder is in home folder.
# Output is to terminal and to log.txt
echo "Time, seconds, blocknum, difficulty" | tee -a log.txt
c=1
prevblock=$(~/zcash/src/zcash-cli getblockcount)
prevseconds=$(date +"%s") # seconds since 1970
until [ $c -gt 5760 ]; do # do for 10 days
block=$(~/zcash/src/zcash-cli getblockcount)
if [ $block != $prevblock ]; then
prevblock=$block
let c=c+1
mytime=$(date +"%T")
myseconds=$(date +"%s") # seconds since 1970
let myseconds=myseconds-prevseconds # seconds since 1970
prevseconds=$(date +"%s")
difficulty=$(~/zcash/src/zcash-cli getdifficulty)
difficulty=${difficulty:0:5}
echo "$mytime $myseconds $block $difficulty" | tee -a log.txt
fi
sleep 10 # seconds
done