r - how to get a ALT0183 point on my x and y axis labels -

i want point (the 1 typing alt0183) between metrics in x , y axis labels.

this want on x-axis: time averaged photon flux (µmoles·m^-2·s^-1)

this want on y-axis: o2 (µmoles·min^-1·mg[chla]^-1)

thusfar, have managed labels without points. second question if know how can keep chla between brackets [] without being made subscript, [chla] stands concentration of chlorophyl a.

this code:

i <- c(30,40,50,60,70,80,100,120,140,160,200)  p <- c(-0.593219548,0.213028562,0.559175617,-0.011447228,0.792755457,0.753335175,1.606054041,1.505984062,2.121712423,2.097202009,1.810391275)  fit = nls(p ~ pm*tanh(a*i/pm), start=list(pm=pm, a=a))  pm=2 a=0.03  par(mgp = c(2.5,1,0))  plot(i,p, xlab=expression(time ~ averaged ~ photon ~ flux ~(µmoles ~ m^{-2} ~ s^{-1})), ylab=expression(o[2] ~ (µmoles ~ min^{-1} ~ mg ~ chla^{-1})), cex.lab=1, main="pi-curve synechocystis batch 1 29/05") 

hopefully bit clear aim in plot.

i fortunately found '·' answer myself!

by putting '%.%' instead of '~' sign, cdot created in axis label.

so code plotting graph became:

plot(i,p, xlab=expression(time ~ averaged ~ photon ~ flux ~(µmoles %.% m^{-2} %.% s^{-1})), ylab=expression(o[2] ~ (µmoles %.% min^{-1} %.% mg ~ chla^{-1})), cex.lab=1, main="pi-curve synechocystis batch 1 29/05") 

however, i'm still looking answer notation of [chla] without becoming subscript.