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<desc>Produced by GNUPLOT 4.0 patchlevel 0 </desc>

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		<text> 0</text>
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	<g transform="translate(59,294)" style="stroke:none; fill:black; font-family:sans-serif; font-size:12.00; text-anchor:end">
		<text> 10</text>
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	<g transform="translate(59,247)" style="stroke:none; fill:black; font-family:sans-serif; font-size:12.00; text-anchor:end">
		<text> 20</text>
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	<g transform="translate(59,200)" style="stroke:none; fill:black; font-family:sans-serif; font-size:12.00; text-anchor:end">
		<text> 30</text>
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	<g transform="translate(59,154)" style="stroke:none; fill:black; font-family:sans-serif; font-size:12.00; text-anchor:end">
		<text> 40</text>
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		<text> 50</text>
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	<g transform="translate(59,60)" style="stroke:none; fill:black; font-family:sans-serif; font-size:12.00; text-anchor:end">
		<text> 60</text>
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	<g transform="translate(59,13)" style="stroke:none; fill:black; font-family:sans-serif; font-size:12.00; text-anchor:end">
		<text> 70</text>
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		<text> 1980</text>
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	<path d='M304,337 L304,9 '></path>
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		<text> 1990</text>
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		<text> 2000</text>
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		<text> 2010</text>
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		<text>Number of references</text>
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		<text>Year of publication</text>
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