Your graph seems to show a fairly consistent HR readings and only a few short complete dropouts. As I said several times optical is not going to be accurate in weight training. Also your average HR in weight training is going to be an over all low number because your HR is only elevated right after a set and if in good condition (and your avatar indicates) your heart should recover quickly. Keep in mind you spend most of the time recovering between sets when weight training. Again HR is unimportant/meaningless metric unless you have a medical condition that needs monitoring when lifting. The only anaerobic activities that benefit from HR monitoring are things like sprints, intervals, HIIT, X-Fit, etc.
Out of curiosity what is it you are looking for in your HR when weight training and what conclusions do you hope to surmise?
I always run mine in Other when weight training but NEVER look at it (at least in the last 1+ year). I use a Garmin 620 in all cardio but also run my

Watch in the appropriate mode. I consider my Garmin the 'official' cardio activity recording and my

Watch as my general overall fitness tracker.
I just did weight training for today (worked back) and keep on HR and subjectively observed the following. Started to also wear my Garmin today as an objective control measure but didn't (too many watches and distractions for weight training and old age forgot it in the car

).
- Lost pulse about 7 times observed but never took more than about 30 seconds to reestablish
- 4 of the 7 times was doing relative exercises and only 3 times with absolute exercises
- Readings were all over the map right after a heavy set (saw as low as 41bpm) as expected
- I recover quickly and reading settled to about correct within a few seconds after a set
- My average HR was recorded at 88bpm and I did a 46 minute workout
- For reference I did road bike 20 miles this morning (and I dislike biking) which will elevate HR when weight training later
EDIT: Just for reference I have a friend who has the Wahoo TICKR BT HR strap and paired with their

Watch. It is a good product at a good price.
http://www.wahoofitness.com/devices/wahoo-tickr-heart-rate-strap-1