It seems that there is no configuration option to set timeline on top of chart.
From example at google site I got the following SVG code for timeline (numbers 1790 and 1800):
<g>
<text text-anchor="middle" x="NaN" y="NaN" font-family="undefined" font-size="0" stroke="none" stroke-width="0" fill-opacity="1" fill="none"></text>
<text text-anchor="middle" x="122.44134100696262" y="144.026" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#000000">1790</text>
<text text-anchor="middle" x="528.1048013844729" y="144.026" font-family="Arial" font-size="13" stroke="none" stroke-width="0" fill="#000000">1800</text>
</g>
It is placed after major SVG code for timeline chart. There is possibility to move it 'manually' using some query/delete/place before chart but that can be error prone.