Syphillis and Gonorrhea – Together in One Graph

I’ve had a few posts on STDs, and in them, I kinda wondered aloud about why syphilis and gonorrhea have evolved differently. It seems odd to me – I figure they’re spread the same way, after all, even if the underlying cause is different. I’m not sure I recall an answer that was completely satisfactory, and just now, putting both series on the same graph, I’m even more confused.

The two series seem to be correlated at the start of the sample, and at the end of the sample… but definitely not in the middle of the sample. What’s going on here?

Best I can figure…

1. Gonorrhea became resistant to treatment, but has since come under control (JackNYC had comments the last time I posted on this subject on the spread of Fluoroquinolone-resistant Neisseria gonorrhoeae, but it seems to me that’s a recent phenomenom)
2. Gonorrhea was misdiagnosed early on – the big rise was merely correct diagnosis. Treatments and avoidance measures that worked for syphilis also worked to bring down the rate of gonorrhea, but it was much higher than had been originally thought.