<timeline>
ImageSize = width:250 height:200
PlotArea = left:40 right:10 top:10 bottom:20
TimeAxis = orientation:horizontal
AlignBars = justify
Colors =
id:gray1 value:gray(0.9)
DateFormat = yyyy
Period = from:1940 till:2010
ScaleMajor = unit:year increment:10 start:1940
PlotData =
bar:1558 color:gray1 width:1
from:start till:end
bar:1038 color:gray1
from:start till:end
bar:519 color:gray1
from:start till:end
bar:0 color:gray1
LineData =
layer:front
points:(62,141)(77,152) color:red width:1
points:(77,152)(100,160) color:red width:1
points:(100,160)(128,161) color:red width:1
points:(128,161)(157,160) color:red width:1
points:(157,160)(185,60) color:red width:1
points:(185,60)(217,74) color:red width:1
</timeline>