R-project: plot 2 zoo objects (price series) that have some date mis-matches

I have 2 zoo objects -

1) Interest rate spread between 10-YR-US-Treasury and 2-YR-US-Treasury
(object name = sprd)

2) S&P 500 index (object name = spy)

str(spy)

?zoo? series from 1976-06-01 to 2011-03-31
?Data: num [1:8791] 99.8 100.2 100.1 99.2 98.6 ...
?Index: Class 'Date' ?num [1:8791] 2343 2344 2345 2346 2349 ...

str(sprd)

?zoo? series from 1976-06-01 to 2011-03-31
?Data: num [1:9088] 0.68 0.71 0.7 0.77 0.79 0.79 0.82 0.86 0.83 0.83 ...
?Index: Class 'Date' ?num [1:9088] 2343 2344 2345 2346 2349 ...


Since there are NA data points in object 'sprd' I created another object
that omits "NA". The name of that object is 'sprdtmp'.

str(sprdtmp)

?zoo? series from 1976-06-01 to 2011-03-31
?Data: atomic [1:8704] 0.68 0.71 0.7 0.77 0.79 0.79 0.82 0.86 0.83 0.83 ...
?- attr(*, "na.action")=Class 'omit' ?int [1:384] 25 70 95 111 118 128 149
190 224 260 ...
?Index: Class 'Date' ?num [1:8704] 2343 2344 2345 2346 2349 ...

I want to plot both time series on the same plot with time/date on the
x-axis and the axis label quarterly (or monthly). One problem is that the
sprdtmp and spy objects do not have the same number of data points as there
are times when the equity markets are closed while the interest rate markets
are open. For the most part the dates overlap. Would this matter if I try to
plot both on the same plot? And how would I go about plotting these objects
in one plot.

The second part of the plot requires that both are on different scales. I
guess I could take a log of the s&p and plot that along with the rate
spread. But it would be nice for future reference how I could plot 2 series
in one plot with 2 difference scales.

I spent all night yesterday and this morning trying various options but I
cant seem to get this to work. I have read through the ?plot, ?plot.zoo and
?axis documentation without any success. I would greatly appreciate if
anyone can point me in the right direction. Thank you kindly.