Comparing trends of Prevalence

I am interested in the prevalence of two conditions within the same population. I have the prevalence of the conditions for a number of years. According to the chi2 test for trend the prevalence for both conditions show a linear trend.

How do I compare the linear trends of the prevalences? Comparing the prevalence of the conditions without taking the log makes not much sense to me.

Is it like lm(log10(prevalence) ~ years*condition).

or do I have to take log10(years) as well.

What would you plot to visualise the trend

  1. plot(years,log10(prevalence))
  2. plot(log10(years),log10(prevalence))
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