I wish to plot a function, but I run into a problem and I cannot find similar questions here. A simplified version of plot is represented by

ParametricPlot[{g,f[x,y]}, {x,a,b},{y,c,d}]

f is quite a tricky expression and cannot be explicitly calculated for unknown x and y. Hence this plot does not work, as I believe Mathematica first explicitly calculates f[x,y] and then thereafter substitutes x and y in order to plot this. However, how can I do this such that f[x,y] substitutes the explicit values for x and y into f, as then the function can be calculated? For example, f[1,2] works absolutely fine, f[x,y] is the problem.

edit:

My code:

` cc = 0.4; cD = 0.1; r = 10.2; Nstar = Floor[r]; rhos1[N_] := (1 - N/r)*Lambda^N* Exp[eps]*Delta; rhos2[N_] := (1 - N/r)*(Lambda*Exp[g])^N* Exp[eps]*Delta*Exp[g*Nstar] rho = Sum[rhos1[n], {n, 1, Nstar}] + Sum[rhos2[n], {n, Nstar + 1, Infinity}]; phi = Sum[n*rhos1[n], {n, 1, Nstar}] + Sum[n*rhos2[n], {n, Nstar + 1, Infinity}]; phi3[Delta_] := Sum[n*(1 - n/r)*\[CapitalLambda]^n* Exp[\[Epsilon]]*Delta, {n, 1, Nstar}] + Sum[n*(1 - n/r)*(Lambda*Exp[g])^n* Exp[eps]*Delta*Exp[g*Nstar], {n, Nstar + 1, Infinity}]; RhoDivided3 = Simplify[rho/Delta] h1 = cD/(RhoDivided3 + Exp[g]); f[eps1_, g1_] := Apply[List, Reduce[cc == ReplaceAll[phi3[h1] + Lambda* Exp[eps], {eps -> eps1, g -> g1}] && Lambda > 0 && Lambda < 0.99999, Lambda], {0, 1}][[2]] Plot[ReplaceAll[phi/rho, {Lambda -> f[-2, g], eps -> -2}], {g, -30, -10}] `

Therefore f is obtained by solving a very complex equation for Lambda, and so cannot be determined as a function of epsilon and g, as explicit values are required.