What should i do if my best fit started at x-axis? How could i write the intercept of the best fit?

If the graph of y = L^(1/2) is a straight line then it is of the form
y = m * L^(1/2) + c
C is the point at which the extension of the line crosses the y axis.
It must cross it somewhere.
It might cross it at the origin which means that c = 0
If it crosses the x axis at some point to the right of the origin then you must extend the graph until it crosses the y axis.
In which case c is a negative number.

m is the gradient of the line

If you are graphing y = L^(1/2) then it's slope is dy/dL = 1/2 L^(-1/2), which means it's not a straight line because the slope is ever changing over the L interval.

However that does not change the observation that y(0) = sqrt(L) = sqrt(0) = 0 is the intercept. Remember the intercept on the YY axis always occurs when x = 0 on the XX axis. In your case when L = 0.