1.  Field of the Invention
This invention relates to a graph displaying method and apparatus and a processing operation monitoring apparatus for displaying measured data, as a graph, on a display screen and more particularly to a technical concept for displaying a transient graph in which the graph to be displayed is varied in coordinate by winding up the displayed graph, by changing a display range or the like. 
2.  Description of the Related Art
Heretofore, a graph displaying method is known in which a new data point of data generated in time series is at all times displayed as an additional graph point on the right end of a display screen while old data points are displayed so as to be moved toward the left end of the display screen.  However, in this conventional method, the graph normally continues moving from the right side to the left side on the display screen so that it is difficult for an operator to observe the graph. 
To this end, a graph displaying apparatus in an effort to solve the conventional problem is disclosed in Japanese Utility Model Laid-Open Publication No.  11751/1986.  In the graph displaying apparatus, a succession of graph points are displayed in order rightwardly from a reference position, located in the midst of a graph display area of a display screen, one graph point at a time being added to the leading end of the graph.  When the latest graph point reaches the end of the graph display area, the entire graph is caused to jump back leftwardly along a time axis in such a manner that the end of the graph meets the reference position.  This process is hereinafter called a "winding-up process".  As subsequent data are generated, an additional succession of graph points representing the subsequent data are displayed in order, restarting off rightwardly from the reference position.  When the right end of the new succession of graph points arrives at the end of the graph display area, then another winding-up process is performed.  Therefore, only when the right end of the graph reaches the end of the graph display area, the graph is instantaneously moved to the reference position, without moving all the time. 
According to this prior concept of the Japanese Publication 11751/1986, it is easy to monitor the data because the graph is kept still until the right end of the graph arrives at the end of the graph display area, whereupon a winding-up process is performed, namely, the graph is instantaneously moved back to the start position.  Since continuity of the graph on the display screen is gone down about this graph winding-up action, the operator's monitoring of the graph would be interrupted due to the graph winding-up process. 
In particular, assume that the reference position is located in the midst of the graph display area in order to leave on the display screen the past data points which for a predetermined time precedes the latest data point at the time immediately after the graph winding-up action.  In this case, the more the old data points are secured, the narrower a part of the graph display area between the reference position and the rightside end of the display screen is obtained.  As a consequence, the winding-up action would occur frequently to thereby break the operator's monitoring at increased frequency.