Turtle draws a curve

Question

I have a textfile with x and y coordinates that I with this program want to calculate a curve with, however, the outcome is a straight line repeating itself The data is imported separately as x and y coordinates. Here is the code that would handle this data

def plotDots(t,x_acc,y_acc):                       
    while x_acc and y_acc:
        t.pu()
        t.goto(x_acc[0],y_acc[0])
        t.pd()
        t.stamp()
        x_acc, y_acc=x_acc[1:], y_acc[1:]

def plotRegression(x,x_acc,y_acc):
    ave_x=sum(x_acc)/len(x_acc)                     #Calculating curve
    ave_y=sum(y_acc)/len(y_acc)
    n=len(x_acc)
    m=((x_acc[0]*y_acc[0])-(n*ave_x*ave_y))/(x_acc[0]**2-(n-x_acc[0]**2))
    y=ave_y+m*(x-ave_x)
    return y

def main():
    t=turtle.Turtle()
    wn=turtle.Screen()
    turtle.setworldcoordinates(-1000,-1000,1000,1000)
    doc=open("labdata.txt")
    line=doc.readline()
    y_acc=[]
    x_acc=[]
    while line:
        values=line.split()
        x_acc=x_acc +[float(values[0])]
        y_acc=y_acc +[float(values[1])]
        line=doc.readline()
    plotDots(t,x_acc,y_acc)
    for x in x_acc:
        t.goto(x, plotRegression(x,x_acc,y_acc))

    doc.close()
    wn.exitonclick()

main()

Answer 1

The way I read your script, you're fitting a linear line to the data. You are then drawing that fit, the linear line, since plotRegression will return the value fitted value y that belongs to your input x (in `t.goto(...).
So it's wonder you see a straight line; you should see that.

Simple use

for x, y in zip(x_acc, y_acc):
  t.goto(x, y)

to draw your curve.

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Note: it makes more sense to calculate the regression once, and then evaluate it for every x. So make a function calcRegression that returns a and b, your fitted parameters, and then a evalRegression that takes input x (either a float or an array of floats) and returns y.
I wouldn't call a function plotRegression if it actually doesn't plot anything.

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If you are going to use this for scientific calculations, you're better off using numpy/scipy/pandas for the calculations, and e.g. matplotlib for the plotting.

If it's just part of practice, no problem, but don't use it for real-world problems, since people have solved this before (and overall, more carefully and more detailed).