Identify significant dates on a line chart

Turf03 , For that One way is to have conditional formatting, which only possible when you have one measure and no legend .

So create a measure like example

Color Date =
var _min =minx(allselected(Date,Date[Year])
return
Switch( true(),
FIRSTNONBLANK('Date'[Year],year(TODAY()))-_min =0 ,"lightgreen",
FIRSTNONBLANK('Date'[Year],year(TODAY()))-_min =0 ,"blue",
"red")

Now first create BAR visual and do conditional formatting and in that use "Field Value" Option

refer

https://radacad.com/dax-and-conditional-formatting-better-together-find-the-biggest-and-smallest-numbers-in-the-column
https://docs.microsoft.com/en-us/power-bi/desktop-conditional-table-formatting#color-by-color-values

Other is have a line with blank and nonblank values based on condition. Refer I use that in this blog

https://community.powerbi.com/t5/Community-Blog/Display-Label-Only-on-the-Last-Data-Point-of-the-Line-Visual-for/ba-p/1253464