zero fractional infectivity plateaus fit with fixtop will draw curves outside bounds

[ckikawa]

Curves fit with fixtop with neutralizing titers outside the bounds of the assay draw curves immediately before the first point (lower bound). This is potentially confusing, since it looks like neutralization is being extrapolated outside the range of dilutions tested in the assay. Curvefits like these are more of a visual issue, since these data and the inferred curves call NT50s as being on the upper or lower bounds, which is the correct quantitative behavior. Example below:

These curves were inferred with fixtop = [0.75,1] and fixslope = [0.8,10]. The bottoms of curves are default fixed to 0. Corresponding curvefits and the raw fractional infectivity values:
frac_infectivity.csv
curvefits.csv