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**Ackbeet** But, more importantly, I think you can get a negatively-sloped dark blue curve. Try a function like this on for size:

$\displaystyle y=\frac{a^{2}}{x-a}-5a.$

Here, the vertical asymptotes are moving to the right with increasing $\displaystyle a,$ and the horizontal asymptotes are moving down with increasing $\displaystyle a.$ But you want the downward movement of the horizontal asymptotes to be faster than the right-ward movement of the vertical asymptote. Hence the 5. I haven't worked this one through, but let me know what you get.