A more or less continuous slide over the last 30 years. My calcs give me a 5% per annum compound decline in cost, not the 14% shown on the chart, although that is not against time but against (I assume) installed capacity (hence the note below the chart of 'learning rate').
This is not as fast as the fall in PV cell costs (+-18% per annum) but still means that costs have fallen 75% in 28 years.
Disclaimer. After nearly 40 years managing money for some of the largest life offices and investment managers in the world, I think I have something to offer. But I can't by law give you advice, and I do make mistakes. Remember: the unexpected sometimes happens. Oddly enough, the expected does too, but all too often it takes longer than you thought it would, or on the other hand happens more quickly than you expected. The Goddess of Markets punishes (eventually) greed, folly, laziness and arrogance. No matter how many years you've served Her. Take care. Be humble. And don't blame me.
BTW, clicking on most charts will produce the original-sized, i.e., bigger version.