how to find y values from smoothing spline curve fit and new x values

Hi there

First of all, i'm unsure whether this question is sufficiently different enough to warrant a new post, if not, apologies and can the moderator merge the questions please.

As background for what i am doing and what i am trying;

https://uk.mathworks.com/matlabcentral/answers/385121-finding-average-value-of-multiple-y-values-for-a-single-x-value

Essentially, I have a graph from a suspension dynamometer, plotting absolute velocity against force which forms a hysteresis loop (Force Vs Velocity.jpg), and from this data i am trying to create a single, averaged line (Force Vs Average Velocity.jpg).

The issue i am having is that i dont have nicely spaced x data. What i have is non-monotonic data which has repetitions, so the number -27.6 is repeated 4 times, with four different values associated with it. Plotting Time against velocity, you get the graph shown in Time Vs Velocity.JPG.

I have attempted curve fitting Time Vs Velocity with a smoothing spline, which has given me a 1x1 cfit named fit_V. What i am trying to do with this is input values for V, such as -300:0.01:300, then somehow, using the curve fit, output the corresponding Time values that would be needed to create those evenly spaced Velocity values.

i have tried;

Velocity_data = -300:0.01:300

y_ouput = fit_V(Velocity_data ) % evaluates as though Velocity data is x data, not y data

y_ouput = feval(fit_V, Velocity_data) % answers to the order of 1E+14, not <350 mm/s

y_output = interp1(Velocity, Time, Velocity_Data) %unique points error

So put simply, how can you find the Y data from user defined x data using cfit or any other method.

Thanks Ross