Not disputing the chart, but have they been observing sunspots that long, or is this something that they’ve measured through other effects?
Yes, sunspots *have* been observed in detail that long. Although methods have improved, there is some degree of allowances for ensuring numbers today match numbers from way back in the apples-to-apples sense - there are different number systems, to make this work.
And, yes, there are other effects. Like thermometer readings from way back and frozen Thames river events.