Edit in JSFiddle

function draw() {
    var canvas = document.getElementById('map');
    canvas.width = 640;
    canvas.height = 360;
    var ctx = canvas.getContext('2d');
    places.forEach(function(place) {
        ctx.rect(
            // turn a longitude value into a screen
            // pixel: the left side of the page is 0, not
            // 180, so add 180 to the value and then
            // scale up by 2 to make the drawing 640px
            // wide
            (~~place[0] + 180) * 2,
            // turn a latitude value into a screen
            // pixel. unlike pixels, latitude ranges
            // from high values on the top to low on the bottom
            // doing 90 - lat flips them.
            (90 - ~~place[1]) * 2, 4, 4);
    });
    ctx.fill();
}

// Big cities in the world, as longitude, latitude pairs
// These are from Natural Earth data.
var places = [[12.453386544971766,41.903282179960115],[12.441770157800141,43.936095834768004],[9.516669472907267,47.13372377429357],[31.19999710971274,-26.466667461352472],[6.130002806227083,49.611660379121076],[158.1499743237623,6.916643696007725],[171.38000017574655,7.103004311216239],[179.21664709402887,-8.516651999041073],[134.62654846699218,7.487396172977981],[-9.652522218250112,26.119166685993378],[7.406913173465057,43.73964568785249],[173.01757082854942,1.338187505624603],[43.240244098693324,-11.70415769566847],[1.51648596050552,42.5000014435459],[-61.51703088544974,10.651997089577264],[30.058585919064114,-1.95164421006325],[31.133334512056365,-26.316650778409212],[31.580025592787308,4.829975198277964],[4.269961302313448,52.08003684397488],[14.51496903347413,46.0552883087945],[17.11698075223461,48.15001832996171],[51.532967894299304,25.286556008906587],[19.266306924118226,42.465972512881706],[79.94999304089748,6.900003884809621],[120.56994258533086,16.42999066056393],[35.75000362014765,-6.183306051766181],[7.466975462482424,46.91668275866772],[-13.200005942222674,27.14998231913546],[21.165984251599866,42.666709614119384],[-61.387012981803366,15.301015644283325],[43.148001667052256,11.595014464255485],[101.7019469797865,2.914019794624551],[135.7480520653204,35.03193814685562],[-16.591701489212596,13.45387646031594],[21.4334614651425,42.00000612290586],[-59.61652673505159,13.10200258275114],[2.61662552756718,6.483310973024231],[29.360006061528395,-3.376087220374643],[-61.212062420279324,13.14827882786841],[-61.00000818036955,14.00197348933034],[-62.71700931969934,17.302030455489387],[57.49999385460973,-20.166638571353246],[-61.74164322607476,12.052633401720414],[50.58305171591019,26.236136290485945],[-61.85003381513866,17.118036518314113],[-56.17299814703597,-34.856095707590725],[1.22081126074562,6.133882930268385],[10.179678099212026,36.80277813623144],[54.36659338259199,24.466683572379907],[58.38329911177465,37.949994933110986],[28.28138173611427,-15.414698409335926],[31.042763572062825,-17.815843835777798],[125.57945593170507,-8.559388408546454],[168.31664058356864,-17.733350404025828],[-87.2194751979415,14.103990759076396],[-58.167028647480606,6.801973692752028],[-21.950014487179544,64.15002361973922],[-72.33798044690553,18.54297045473237],[32.581377667121046,0.318604813383331],[-55.16703088542437,5.835030129922586],[2.114710186530374,13.51865181050647],[68.77387935270173,38.56003521631658],[-57.643451027901335,-25.294457117057675],[-86.27043751890119,12.154962438756115],[-13.236161599012746,8.471957271098177],[73.16468862105955,33.70194180895959],[85.31469635222788,27.718637772477223],[26.22991288117737,-29.119993877378704],[28.22748321723384,-25.704974695184433],[147.19250362059358,-9.464707825867777],[159.94976573360566,-9.437994295089595],[-79.53498301041077,8.969963049094872],[-6.83640815612614,34.02530731107282],[28.85771113965143,47.005023619670624],[32.58721710397009,-25.953331628778983],[45.36473175245874,2.068627192947531],[58.593312132608844,23.613324807728134],[79.85775060925641,6.931965758182116],[106.91466990374653,47.91861925856074],[174.78327429127694,-41.29997393927641],[17.08354610054181,-22.570006084383806],[7.53138214293233,9.085279007754195],[-15.598360841320755,11.865023822980561],[35.93135406687412,31.951971105827454],[25.3166352932829,54.68336631175862],[24.099965371403187,56.95002382316096],[74.58325836390367,42.875025305090105],[27.483273069984477,-29.316674378681626],[47.51467804152986,-18.914691492032148],[-78.5019969671124,-0.213042322035562],[-84.08599721127536,9.936958288356607],[-89.2049870794599,13.711947505494038],[-76.7674337136691,17.977076623830897],[15.047202455462298,12.115042394810644],[8.783277545821136,3.750015278026183],[38.933323525759306,15.333339252681924],[15.999994668245677,45.80000673327254],[24.72804072947855,59.43387737948592],[33.78330195998353,-13.983295065469179],[-90.52891143656154,14.623080521448173],[9.457965045823698,0.385388609718518],[178.44170731537986,-18.133015931371233],[-71.62295949149893,-33.045818607974184],[-15.975340414890013,18.086427021247516],[-8.001984963249697,12.65196052632325],[35.507762351377664,33.8739209756269],[44.788849590997984,41.72695584707759],[71.427774209483,51.18112530425759],[102.59998002015476,17.96669272762739],[15.282743633848668,-4.257239913197509],[-13.682180886123945,9.53346870502179],[-5.275502564912301,6.818380960004617],[-75.70196115980951,45.41864265536043],[20.466044822020535,44.82059130444674],[114.93328405666227,4.883331114619239],[-65.25951562667564,-19.04097084673947],[-88.76707299981655,17.252033507246892],[18.558288125287277,4.366644306349087],[11.514704896854425,3.868646520754112],[19.81888301461521,41.327540709491586],[44.51160553175208,40.18309659414189],[49.860271303257775,40.39721789134302],[104.91468862118643,11.55197598855841],[-68.15193104910219,-16.49602775504337],[2.518044740568598,6.401954422782467],[23.314708152110086,42.68529528393054],[27.56468129665825,53.901923295043105],[89.63901403703,27.4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draw();
<canvas id='map'></canvas>