Please check your web server log file. If it says php was unable to allocate memory, then you'll have to increase your max memory_limit (php.ini).

Otherwise, post the error message here, so we can investigate further.

Seems to be that this is wanted.

https://github.com/piwik/piwik/blob/master/plugins/VisitsSummary/Controller.php:

76          if($view->period != 'year')
77          {
78              $view->urlSparklineNbUniqVisitors     = $this->getUrlSparkline( 'getEvolutionGraph', array('columns' => array('nb_uniq_visitors')));
79              $view->nbUniqVisitors = $dataRow->getColumn('nb_uniq_visitors');
80          }

I guess, it would make sense to display it for day only, 'cause the value is inaccurate for other periods.

(In [2439]) refs #1464 remove year restriction

Evolution graph should now be shown in every period.

Should we add a note to the text (if period is not day) explaining that this value is a sum of all daily values?

I think it looks good like this :)

(In [2448]) refs #1464 if-statement in template no longer required