Plot the logarithm of the absolute value of y multiplied by sign of y, and the change labels:
x = linspace(0,3,10000);
y = 1e6*exp(-x).*(sin(10*x)-0.5);
plot(x,y)
figure
Y = sign(y).*log10(abs(y));
plot(x,Y,'.')
yl = get(gca,'ytick');
set(gca,'yticklabel',sign(yl).*10.^abs(yl))
