Ghost point appearing on student-entered scatterplot

Questions
1

My goal: Students enter a set of ordered pairs that appear on a graph as they are entered. With a click of a button, Desmos draws the graph of a parabola through those points.

The problem: On preview, a dot for the point (3,1) appears before any points are entered. Entering information in the top row appears to change the location of this dot. Additional dots appear as expected.

I have no idea why this initial point is appearing! Here is a copy of the slide in question: Ghost Points? • Activity Builder by Desmos

2

Looks like it’s probably from the regression. Maybe this?
(X_1,Y_1 {length(Y_1)>1})

3

If I add that in, the graph won’t show whatever I put in the first row until something else is added in the second. For whatever reason, the point is affected by what is entered in the first row. Oddly, adding a value for x and then deleting it will move the point and leave it there, while adding a value for y and then deleting it will move the point, then move it back to (0,1)

4

Buncha ways to fix this one. Daniel’s got it on the right track. The regression is treating X1 and Y1 as free variables until CL determines its a list. You can keep the regression from firing until something has been entered into both the x and y column.

number(L_x): length(inverseTable.columnNumericValues(1))
number(L_y): length(inverseTable.columnNumericValues(2))

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5

That seems to have done it! Thanks Jay. I’m not sure how I would have figured that out on my own…

6

Regressions are tricky!