meridian label for geographical map with mercator projection

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I was trying to plot sea surface temperature data with lat/lon coordinates on a map with mercator projection. The data is a region between 40S to 40N in latitude and 140E to 20W in longitude. The data named testdata.mat can be downloaded at data file. I would like to set the origin to be 180E in longitude. The question is how I can make the meridian label correctly?. Here are the steps I tried:
Step 1: Here, I setup the projection and ploted the data. I got the following figure named "plot1.png".
% map projection
ax=axesm('MapProjection', 'mercator', 'Frame', 'off');
setm(ax,'meridianlabel', 'on')
setm(ax, 'parallellabel', 'on')
setm(ax, 'Origin', [0 180 0]);
lat_lim = [-40, 40];
lon_lim = [140, -20];
setm(ax, 'MapLatLimit', lat_lim, 'MapLonLimit', lon_lim);
hold on;
scatterm(locs(:,1), locs(:,2), 50, Z+273.15, '.');
Step 2: Here I colored the land to be gray, and adjust the the position of labels. I got the following figure named "plot2.png". As you can see that the labels with longitude less than 0 are all labeled as positive longitude easterward, i.e.,280E, 340E. I would like to label them as 80W, 20W. In addition, I would also like to label "180E" as "180".
load coast
plotm(lat, long, 'k-')
lightgrey = [.8,.8,.8];
setm(ax,'mlabelparallel',-90) % change the position of lon labels
%setm(ax, 'LabelFormat', 'none');
setm(ax, 'Grid', 'off')
setm(ax, 'LabelRotation', 'on');
setm(ax, 'MLabelLocation', [140, 180, 360-80, 360-20]);
setm(ax, 'PLabelLocation', [-40, 0, 40]);
hc=colorbar('peer', ax, 'East');
set(ax, 'CLim', [270 305]);
set(hc, 'Position', [0.88, 0.348, 0.025, 0.34], 'YAxisLocation','right')
title(ax, 'SST');
axis off

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