Showing the rise and decay of topics over the last two months (= default setting, which can be changed using the date selector), the trend chart provides the following time series:

• Frequency (Total, Positive and Negative). The displayed frequency values represent the number of occurrences in the last seven days. Selecting positive or negative frequency not only changes the data displayed in the chart, but also activates a global sentiment filter that affects the entire portal. Once activated, the footer of the portal shows the current filter status. Selecting total frequency deactivates the sentiment filter.
• Sentiment (Mean, Standard Deviation). While the mean value measures average sentiment towards a term within a sample, standard deviation reflects how contested a particular topic is (the term ‘oil spill’, for example, tends to have a low standard deviation since everyone agrees on its negative connotation).

Term Selection

The category drop-down menu enables users to select a category and show the five highest-ranking keywords within this category, or pick a set of predefined terms. Clicking on a keyword in the legend triggers a full-text query; clicking on the colored series marker hides the corresponding series from the chart. The term selection drop-down menu in the lower right corner allows accessing additional keywords computed for the selected period, and select an arbitrary set of these keywords to be displayed in the chart.

Display Features

The vertical axis is rescaled automatically, a feature that is particularly useful if one keyword dominates the coverage and therefore obscures the distributions of the other keywords.

Hovering above a data point displays the associated keywords. This mouse-over effect identifies topical trends and shows their impact on the individual peaks in the chart. Clicking on a data point triggers a search for this particular topic cluster in the preceding week.