如何在 gnuplot 中绘制到数据点的切线？

Question

I am wondering how to draw a tangent line to a data point from csv data file in gnuplot. Thanks for help

I tried some sort of fitting but it does not work.

Answer 1

How do you specify your data point for the tangent? By x-value? Are there unique y-values for all x-values? You give very little information and no example data, so I assume something.

Get the current and previous x-values into the variables x1 and x0 , respectively. If your x-value for the tangent xt is between x0 and x1 calculate the slope m and the y-axis-offset c . Your tangent line then is y = m*x + c .

If you want to plot a tangent to a function you could use this approach .

Script:

### plot a tangent to a dataset
reset session

# create some test data
$RawData <<EOD
 0   0
 1   1
 2   4
 3   2
 4   1
 5   2
EOD
set table $Data
    plot $RawData u 1:2 smooth mcsplines
unset table

xt = 2.1

getTangent(colX,colY) = (x0=x1, y0=y1, x1=column(colX), y1=column(colY), \
                    sgn(xt-x0)!=sgn(xt-x1) ? (m=(y1-y0)/(x1-x0), c=y0-x0*m) : 0)

plot x1=y1=NaN $Data u 1:(getTangent(1,2),$2) w l lc "red" ti "Data", \
        '+' u 1:(m*$1+c) w l lc "blue" ti sprintf("Tangent at x=%g",xt)
### end of script

Result: