# Ghost point appearing on student-entered scatterplot

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? â¢ Activity Builder by Desmos

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

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)

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))

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

Regressions are tricky!