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

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		<text> 0</text>
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		<text> 2</text>
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<g style="fill:none; color:gray; stroke:currentColor; stroke-width:1.00">
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<g style="fill:none; color:black; stroke:currentColor; stroke-width:1.00">
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	<g transform="translate(59,259)" style="stroke:none; fill:black; font-family:sans-serif; font-size:12.00; text-anchor:end">
		<text> 4</text>
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	<g transform="translate(59,218)" style="stroke:none; fill:black; font-family:sans-serif; font-size:12.00; text-anchor:end">
		<text> 6</text>
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<g style="fill:none; color:black; stroke:currentColor; stroke-width:0.20">
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	<path d='M76,173 L67,173 M782,173 L791,173 '></path>
	<g transform="translate(59,177)" style="stroke:none; fill:black; font-family:sans-serif; font-size:12.00; text-anchor:end">
		<text> 8</text>
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		<text> 10</text>
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	<g transform="translate(59,95)" style="stroke:none; fill:black; font-family:sans-serif; font-size:12.00; text-anchor:end">
		<text> 12</text>
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		<text> 14</text>
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		<text> 16</text>
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	<path d='M167,337 L167,9 '></path>
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		<text> 1970</text>
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	<path d='M348,337 L348,9 '></path>
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<g style="fill:none; color:gray; stroke:currentColor; stroke-width:1.00">
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		<text> 1980</text>
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<g style="fill:none; color:gray; stroke:currentColor; stroke-width:1.00">
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	<path d='M529,337 L529,346 M529,9 L529,0 '></path>
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		<text> 1990</text>
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		<text> 2000</text>
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		<text>Number of references</text>
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		<text>Year of publication</text>
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