The story is recounted in a June 8, 1932 edition of the Viroqua Censor (here.)  The article begins:

In the Court House yard, almost under the shadow of the County Capitol, beneath a sapling rests a marker placed there by the late Dr. Porter, two years ago, that attracts attention from almost every person who passes the spot.  The plain simplicity of the marker, "June 1, 1888," imprinted thereon, provokes very frequent inquiry as to its significance?  It marks the fatal place where Andrew Grandstaff was executed at the hands of a determined band of men bent on revenge, on the date indicated.

I had noticed that marker and also wondered why it was there.  At the time I thought it might have been placed when the tree was planted, but thinking about it, a 130 year old Douglas Fir would probably be larger.  Actually the hanging tree was a hickory as described in this excerpt from a DNR publication:

Andrew Grandstaff, who murdered four people, two of them children, was hanged from a tree on the Vernon County Courthouse lawn before he could be brought to trial. It happened on the night of Viroqua High School's graduation--June 28, 1888.[sic]

Afterwards the tree, virtually destroyed by souvenir hunters, had to be taken down. A portion of it, as well as the victim's clothing, is still on display at the Vernon County Historical Society in Viroqua.

Towards the end of the Censor article the author pauses to reflect on this watershed event for all involved:

No crime more revolting or atrocious was ever committed, and if mob violence was ever justifiable it was in this very case.  Notwithstanding the enormity of the crime, the Censor regrets exceedingly that this stain has been laid at the door of our people.  To be sure, no better citizens live in this county than lent a hand in the affair, and they, at the time, no doubt thought their act justifiable.

How indebted we are to that doctor who decided that losing this story to history was not acceptable. I wonder what concerns in 1930 led him to want to remind us of our potential for cruelty and injustice.